Methods for driving bistable electro-optic displays, and apparatus for use therein

ABSTRACT

A bistable electro-optic display has a plurality of pixels, each of which is capable of displaying at least three gray levels. The display is driven by a method comprising: storing a look-up table containing data representing the impulses necessary to convert an initial gray level to a final gray level; storing data representing at least an initial state of each pixel of the display; receiving an input signal representing a desired final state of at least one pixel of the display; and generating an output signal representing the impulse necessary to convert the initial state of said one pixel to the desired final state thereof, as determined from said look-up table. The invention also provides a method for reducing the remnant voltage of an electro-optic display.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation of copending application Ser. No.14/726,489, filed May 30, 2015 (Publication No. 2015/0262551), which isitself a continuation of application Ser. No. 12/780,998, filed May 17,2010 (Publication No. 2010/0220121, now abandoned) which is itself acontinuation of copending application Ser. No. 11/307,886, filed Feb.27, 2006 (Publication No. 2006/0139310, now U.S. Pat. No. 7,733,335,issued Jun. 8, 2010), which is itself a continuation of application Ser.No. 10/065,795, filed Nov. 20, 2002 (Publication No. 2003/0137521, nowU.S. Pat. No. 7,012,600, issued Mar. 14, 2006), which claims priorityfrom the following Provisional applications: (a) Ser. No. 60/319,007,filed Nov. 20, 2001; (b) Ser. No. 60/319,010, filed Nov. 21, 2001; (c)Ser. No. 60/319,034, filed Dec. 18, 2001; (d) Ser. No. 60/319,037, filedDec. 20, 2001; and (e) Ser. No. 60/319,040, filed Dec. 21, 2001. Theaforementioned application Ser. No. 10/065,795 is also acontinuation-in-part of copending application Ser. No. 09/561,424, filedApr. 28, 2000 (now U.S. Pat. No. 6,531,997, issued Mar. 11, 2003), whichis itself a continuation-in-part of copending application Ser. No.09/520,743, filed Mar. 8, 2000 (now U.S. Pat. No. 6,504,524, issued Jan.7, 2003). Application Ser. No. 09/561,424 also claims priority fromApplication Ser. No. 60/131,790, filed Apr. 30, 1999. The entirecontents of the aforementioned applications are herein incorporated byreference.

This application is also related to copending application Ser. No.11/160,455, filed Jun. 24, 2005 (now U.S. Pat. No. 7,312,794), which isa divisional of the aforementioned application Ser. No. 10/065,795.

BACKGROUND OF INVENTION

This invention relates to methods for driving bistable electro-opticdisplays, and to apparatus for use in such methods. More specifically,this invention relates to driving methods and apparatus controller whichare intended to enable more accurate control of gray states of thepixels of an electro-optic display. This invention also relates to amethod which enables long-term direct current (DC) balancing of thedriving impulses applied to an electrophoretic display. This inventionis especially, but not exclusively, intended for use with particle-basedelectrophoretic displays in which one or more types of electricallycharged particles are suspended in a liquid and are moved through theliquid under the influence of an electric field to change the appearanceof the display.

In one aspect, this invention relates to apparatus which enableselectro-optic media which are sensitive to the polarity of the appliedfield to be driven using circuitry intended for driving liquid crystaldisplays, in which the liquid crystal material is not sensitive topolarity.

The term “electro-optic” as applied to a material or a display, is usedherein in its conventional meaning in the imaging art to refer to amaterial having first and second display states differing in at leastone optical property, the material being changed from its first to itssecond display state by application of an electric field to thematerial. Although the optical property is typically color perceptibleto the human eye, it may be another optical property, such as opticaltransmission, reflectance, luminescence or, in the case of displaysintended for machine reading, pseudo-color in the sense of a change inreflectance of electromagnetic wavelengths outside the visible range.

The term “gray state” is used herein in its conventional meaning in theimaging art to refer to a state intermediate two extreme optical statesof a pixel, and does not necessarily imply a black-white transitionbetween these two extreme states. For example, several of the patentsand published applications referred to below describe electrophoreticdisplays in which the extreme states are white and deep blue, so that anintermediate “gray state” would actually be pale blue. Indeed, asalready mentioned the transition between the two extreme states may notbe a color change at all.

The terms “bistable” and “bistability” are used herein in theirconventional meaning in the art to refer to displays comprising displayelements having first and second display states differing in at leastone optical property, and such that after any given element has beendriven, by means of an addressing pulse of finite duration, to assumeeither its first or second display state, after the addressing pulse hasterminated, that state will persist for at least several times, forexample at least four times, the minimum duration of the addressingpulse required to change the state of the display element. It is shownin copending application Ser. No. 10/063,236, filed Apr. 2, 2002(Publication No. 2002/0180687; see also the corresponding Internationalapplication Publication No. WO 02/079869) that some particle-basedelectrophoretic displays capable of gray scale are stable not only intheir extreme black and white states but also in their intermediate graystates, and the same is true of some other types of electro-opticdisplays. This type of display is properly called “multi-stable” ratherthan bistable, although for convenience the term “bistable” may be usedherein to cover both bistable and multi-stable displays.

The term “gamma voltage” is used herein to refer to external voltagereferences used by drivers to determine voltages to be applied to pixelsof a display. It will be appreciated that a bistable electro-opticmedium does not display the type of one-to-one correlation betweenapplied voltage and optical state characteristic of liquid crystals, theuse of the term “gamma voltage” herein is not precisely the same as withconventional liquid crystal displays, in which gamma voltages determineinflection points in the voltage level/output voltage curve.

The term “impulse” is used herein in its conventional meaning of theintegral of voltage with respect to time. However, some bistableelectro-optic media act as charge transducers, and with such media analternative definition of impulse, namely the integral of current overtime (which is equal to the total charge applied) may be used. Theappropriate definition of impulse should be used, depending on whetherthe medium acts as a voltage-time impulse transducer or a charge impulsetransducer.

Several types of bistable electro-optic displays are known. One type ofelectro-optic display is a rotating bichromal member type as described,for example, in U.S. Pat. Nos. 5,808,783; 5,777,782; 5,760,761;6,054,071 6,055,091; 6,097,531; 6,128,124; 6,137,467; and 6,147,791(although this type of display is often referred to as a “rotatingbichromal ball” display, the term “rotating bichromal member” ispreferred as more accurate since in some of the patents mentioned abovethe rotating members are not spherical). Such a display uses a largenumber of small bodies (typically spherical or cylindrical) which havetwo or more sections with differing optical characteristics, and aninternal dipole. These bodies are suspended within liquid-filledvacuoles within a matrix, the vacuoles being filled with liquid so thatthe bodies are free to rotate. The appearance of the display is changedto applying an electric field thereto, thus rotating the bodies tovarious positions and varying which of the sections of the bodies isseen through a viewing surface.

Another type of electro-optic medium uses an electrochromic medium, forexample an electrochromic medium in the form of a nanochromic filmcomprising an electrode formed at least in part from a semi-conductingmetal oxide and a plurality of dye molecules capable of reversible colorchange attached to the electrode; see, for example O'Regan, B., et al.,Nature 1991, 353, 737; and Wood, D., Information Display, 18(3), 24(March 2002). See also Bach, U., et al., Adv. Mater., 2002, 14(11), 845.Nanochromic films of this type are also described, for example, in U.S.Pat. No. 6,301,038, International application Publication No. WO01/27690, and in copending application Ser. Nos. 60/365,368; 60/365,369;60/365,385 and 60/365,365, all filed Mar. 18, 2002, application Ser.Nos. 60/319,279; 60/319,280; and 60/319,281, all filed May 31, 2002; andApplication Ser. No. 60/319,438, filed Jul. 31, 2002.

Another type of electro-optic display, which has been the subject ofintense research and development for a number of years, is theparticle-based electrophoretic display, in which a plurality of chargedparticles move through a suspending fluid under the influence of anelectric field. Electrophoretic displays can have attributes of goodbrightness and contrast, wide viewing angles, state bistability, and lowpower consumption when compared with liquid crystal displays.Nevertheless, problems with the long-term image quality of thesedisplays have prevented their widespread usage. For example, particlesthat make up electrophoretic displays tend to settle, resulting ininadequate service-life for these displays.

Numerous patents and applications assigned to or in the names of theMassachusetts Institute of Technology (MIT) and E Ink Corporation haverecently been published describing encapsulated electrophoretic media.Such encapsulated media comprise numerous small capsules, each of whichitself comprises an internal phase containing electrophoretically-mobileparticles suspended in a liquid suspension medium, and a capsule wallsurrounding the internal phase. Typically, the capsules are themselvesheld within a polymeric binder to form a coherent layer positionedbetween two electrodes. Encapsulated media of this type are described,for example, in U.S. Pat. Nos. 5,930,026; 5,961,804; 6,017,584;6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851; 6,130,773;6,130,774; 6,172,798; 6,177,921; 6,232,950; 6,249,271; 6,252,564;6,262,706; 6,262,833; 6,300,932; 6,312,304; 6,312,971; 6,323,989;6,327,072; 6,376,828; 6,377,387; 6,392,785; 6,392,786; 6,413,790;6,422,687; 6,445,374; 6,445,489; and 6,459,418; and U.S. PatentApplications Publication Nos. 2001/0045934; 2002/0019081; 2002/0021270;2002/0053900; 2002/0060321; 2002/0063661; 2002/0063677; 2002/0090980;2002/106847; 2002/0113770; 2002/0130832; 2002/0131147; and 2002/0154382,and International Applications Publication Nos. WO 99/53373; WO99/59101; WO 99/67678; WO 00/05704; WO 00/20922; WO 00/38000; WO00/38001; WO 00/36560; WO 00/20922; WO 00/36666; WO 00/67110; WO00/67327; WO 01/07961; WO 01/08241; WO 01/17029; and WO 01/17041.

Many of the aforementioned patents and applications recognize that thewalls surrounding the discrete microcapsules in an encapsulatedelectrophoretic medium could be replaced by a continuous phase, thusproducing a so-called polymer-dispersed electrophoretic display in whichthe electrophoretic medium comprises a plurality of discrete droplets ofan electrophoretic fluid and a continuous phase of a polymeric material,and that the discrete droplets of electrophoretic fluid within such apolymer-dispersed electrophoretic display may be regarded as capsules ormicrocapsules even though no discrete capsule membrane is associatedwith each individual droplet; see for example, WO 01/02899, at page 10,lines 6-19. See also copending application Ser. No. 09/683,903, filedFeb. 28, 2002 (now U.S. Pat. No. 6,866,760), and the correspondingInternational Application PCT/US02/06393. Accordingly, for purposes ofthe present application, such polymer-dispersed electrophoretic mediaare regarded as sub-species of encapsulated electrophoretic media.

An encapsulated electrophoretic display typically does not suffer fromthe clustering and settling failure mode of traditional electrophoreticdevices and provides further advantages, such as the ability to print orcoat the display on a wide variety of flexible and rigid substrates.(Use of the word “printing” is intended to include all forms of printingand coating, including, but without limitation: pre-metered coatingssuch as patch die coating, slot or extrusion coating, slide or cascadecoating, curtain coating; roll coating such as knife over roll coating,forward and reverse roll coating; gravure coating; dip coating; spraycoating; meniscus coating; spin coating; brush coating; air knifecoating; silk screen printing processes; electrostatic printingprocesses; thermal printing processes; ink jet printing processes; andother similar techniques.) Thus, the resulting display can be flexible.Further, because the display medium can be printed (using a variety ofmethods), the display itself can be made inexpensively.

A related type of electrophoretic display is a so-called “microcellelectrophoretic display”. In a microcell electrophoretic display, thecharged particles and the suspending fluid are not encapsulated withinmicrocapsules but instead are retained within a plurality of cavitiesformed within a carrier medium, typically a polymeric film. See, forexample, International Applications Publication No. WO 02/01281, andpublished US Application No. 2002-0075556, both assigned to SipixImaging, Inc.

The bistable or multi-stable behavior of particle-based electrophoreticdisplays, and other electro-optic displays displaying similar behavior,is in marked contrast to that of conventional liquid crystal (“LC”)displays. Twisted nematic liquid crystals act are not bi- ormulti-stable but act as voltage transducers, so that applying a givenelectric field to a pixel of such a display produces a specific graylevel at the pixel, regardless of the gray level previously present atthe pixel. Furthermore, LC displays are only driven in one direction(from non-transmissive or “dark” to transmissive or “light”), thereverse transition from a lighter state to a darker one being effectedby reducing or eliminating the electric field. Finally, the gray levelof a pixel of an LC display is not sensitive to the polarity of theelectric field, only to its magnitude, and indeed for technical reasonscommercial LC displays usually reverse the polarity of the driving fieldat frequent intervals.

In contrast, bistable electro-optic displays act, to a firstapproximation, as impulse transducers, so that the final state of apixel depends not only upon the electric field applied and the time forwhich this field is applied, but also upon the state of the pixel priorto the application of the electric field. Furthermore, it has now beenfound, at least in the case of many particle-based electro-opticdisplays, that the impulses necessary to change a given pixel throughequal changes in gray level (as judged by eye or by standard opticalinstruments) are not necessarily constant, nor are they necessarilycommutative. For example, consider a display in which each pixel candisplay gray levels of 0 (white), 1, 2 or 3 (black), beneficially spacedapart. (The spacing between the levels may be linear in percentagereflectance, as measured by eye or by instruments but other spacings mayalso be used. For example, the spacings may be linear in L*, or may beselected to provide a specific gamma; a gamma of 2.2 is often adoptedfor monitors, and where the present displays are be used as areplacement for a monitor, use of a similar gamma may be desirable.) Ithas been found that the impulse necessary to change the pixel from level0 to level 1 (hereinafter for convenience referred to as a “0-1transition”) is often not the same as that required for a 1-2 or 2-3transition. Furthermore, the impulse needed for a 1-0 transition is notnecessarily the same as the reverse of a 0-1 transition. In addition,some systems appear to display a “memory” effect, such that the impulseneeded for (say) a 0-1 transition varies somewhat depending upon whethera particular pixel undergoes 0-0-1, 1-0-1 or 3-0-1 transitions. (Where,the notation “x-y-z”, where x, y, and z are all optical states 0, 1, 2,or 3 denotes a sequence of optical states visited sequentially in time.)Although these problems can be reduced or overcome by driving all pixelsof the display to one of the extreme states for a substantial periodbefore driving the required pixels to other states, the resultant“flash” of solid color is often unacceptable; for example, a reader ofan electronic book may desire the text of the book to scroll down thescreen, and may be distracted, or lose his place, if the display isrequired to flash solid black or white at frequent intervals.Furthermore, such flashing of the display increases its energyconsumption and may reduce the working lifetime of the display. Finally,it has been found that, at least in some cases, the impulse required fora particular transition is affected by the temperature and the totaloperating time of the display, and by the time that a specific pixel hasremained in a particular optical state prior to a given transition, andthat compensating for these factors is desirable to secure accurate grayscale rendition.

In one aspect, this invention seeks to provide a method and a controllerthat can provide accurate gray levels in an electro-optic displaywithout the need to flash solid color on the display at frequentintervals.

Furthermore, as will readily be apparent from the foregoing discussion,the drive requirements of bistable electro-optic media render unmodifieddrivers designed for driving active matrix liquid crystal displays(AMLCD's) unsuitable for use in bistable electro-optic media-baseddisplays. However, such AMLCD drivers are readily availablecommercially, with large permissible voltage ranges and high pin-countpackages, on an off-the-shelf basis, and are inexpensive, so that suchAMLCD drives are attractive for drive bistable electro-optic displays,whereas similar drivers custom designed for bistable electro-opticmedia-based displays would be substantially more expensive, and wouldinvolve substantial design and production time. Accordingly, there arecost and development time advantages in modifying AMLCD drivers for usewith bistable electro-optic displays, and this invention seeks toprovide a method and modified driver which enables this to be done.

Also, as already noted, this invention relates to methods for drivingelectrophoretic displays which enable long-term DC-balancing of thedriving impulses applied to the display. It has been found thatencapsulated and other electrophoretic displays need to be driven withaccurately DC-balanced waveforms (i.e., the integral of current againsttime for any particular pixel of the display should be held to zero overan extended period of operation of the display) to preserve imagestability, maintain symmetrical switching characteristics, and providethe maximum useful working lifetime of the display. Conventional methodsfor maintaining precise DC-balance require precision-regulated powersupplies, precision voltage-modulated drivers for gray scale, andcrystal oscillators for timing, and the provision of these and similarcomponents adds greatly to the cost of the display.

Furthermore, even with the addition of such expensive components, trueDC balance is still not obtained. Empirically it has been found thatmany electrophoretic media have asymmetric current/voltage (UV curves);it is believed, although the invention is in no way limited by thisbelief, that these asymmetric curves are due to electrochemical voltagesources within the media. These asymmetric curves mean that the currentwhen the medium is addressed to one extreme optical state (say black) isnot the same as when the medium is addressed to the opposed extremeoptical state (say white), even when the voltage is carefully controlledto be precisely the same in the two cases.

It has now been found that the extent of DC imbalance in anelectrophoretic medium used in a display can be ascertained by measuringthe open-circuit electrochemical potential (hereinafter for conveniencecalled the “remnant voltage” of the medium. When the remnant voltage ofa pixel is zero, it has been perfectly DC balanced. If its remnantvoltage is positive, it has been DC unbalanced in the positivedirection. If its remnant voltage is negative, it has been DC unbalancedin the negative direction. This invention uses remnant voltage data tomaintain long-term DC-balancing of the display.

SUMMARY OF INVENTION

Accordingly, in one aspect, this invention provides a method of drivinga bistable electro-optic display having a plurality of pixels, each ofwhich is capable of displaying at least three gray levels (as isconventional in the display art, the extreme black and white states areregarded as two gray levels for purposes of counting gray levels). Themethod comprises:

-   -   storing a look-up table containing data representing the        impulses necessary to convert an initial gray level to a final        gray level;    -   storing data representing at least an initial state of each        pixel of the display;    -   receiving an input signal representing a desired final state of        at least one pixel of the display; and    -   generating an output signal representing the impulse necessary        to convert the initial state of said one pixel to the desired        final state thereof, as determined from said look-up table.

This method may hereinafter for convenience be referred to as the“look-up table method” of the present invention.

This invention also provides a device controller for use in such amethod. The controller comprises:

-   -   storage means arranged to store both a look-up table containing        data representing the impulses necessary to convert an initial        gray level to a final gray level, and data representing at least        an initial state of each pixel of the display;    -   input means for receiving an input signal representing a desired        final state of at least one pixel of the display;    -   calculation means for determining, from the input signal, the        stored data representing the initial state of said pixel, and        the look-up table, the impulse required to change the initial        state of said one pixel to the desired final state; and output        means for generating an output signal representative of said        impulse.

This invention also provides a method of driving a bistableelectro-optic display having a plurality of pixels, each of which iscapable of displaying at least three gray levels. The method comprises:

-   -   storing a look-up table containing data representing the        impulses necessary to convert an initial gray level to a final        gray level;    -   storing data representing at least an initial state of each        pixel of the display;    -   receiving an input signal representing a desired final state of        at least one pixel of the display; and    -   generating an output signal representing the impulse necessary        to convert the initial state of said one pixel to the desired        final state thereof, as determined from said look-up table, the        output signal representing the period of time for which a        substantially constant drive voltage is to be applied to said        pixel.

This invention also provides a device controller for use in such amethod. The controller comprises:

-   -   storage means arranged to store both a look-up table containing        data representing the impulses necessary to convert an initial        gray level to a final gray level, and data representing at least        an initial state of each pixel of the display;    -   input means for receiving an input signal representing a desired        final state of at least one pixel of the display;    -   calculation means for determining, from the input signal, the        stored data representing the initial state of said pixel, and        the look-up table, the impulse required to change the initial        state of said one pixel to the desired final state; and output        means for generating an output signal representative of said        impulse, the output signal representing the period of time for        which a substantially constant drive voltage is to be applied to        said pixel.

In another aspect, this invention provides a device controller for usein the method of the present invention. The controller comprises:

-   -   storage means arranged to store both a look-up table containing        data representing the impulses necessary to convert an initial        gray level to a final gray level, and data representing at least        an initial state of each pixel of the display;    -   input means for receiving an input signal representing a desired        final state of at least one pixel of the display;    -   calculation means for determining, from the input signal, the        stored data representing the initial state of said pixel, and        the look-up table, the impulse required to change the initial        state of said one pixel to the desired final state; and output        means for generating an output signal representative of said        impulse, the output signal representing a plurality of pulses        varying in at least one of voltage and duration, the output        signal representing a zero voltage after the expiration of a        predetermined period of time.

In another aspect, this invention provides a driver circuit havingoutput lines arranged to be connected to drive electrodes of anelectro-optic display. This driver circuit has first input means forreceiving a plurality of (n+1) bit numbers representing the voltage andpolarity of signals to be placed on the drive electrodes; and secondinput means for receiving a clock signal. Upon receipt of the clocksignal, the driver circuit displays the selected voltages on its outputlines. In one preferred form of this driver circuit, the selectedvoltages may be any one of 2 n discrete voltages between R and R+V,where R is a predetermined reference voltage (typically the voltage of acommon front electrode in an active matrix display, as described in moredetail below), and V is the maximum difference from the referencevoltage which the driver circuit can assert, or any one of 2 n discretevoltages between R and R−V. These selected voltages may be linearlydistributed over the range of R±V, or may be distributed in a non-linearmanner; the non-linearity may be controlled by two or more gammavoltages placed within the specified range, each gamma voltage defininga linear regime between that gamma voltage and the adjacent gamma orreference voltage.

In another aspect, this invention provides a driver circuit havingoutput lines arranged to be connected to drive electrodes of anelectro-optic display. This driver circuit has first input means forreceiving a plurality of 2-bit numbers representing the voltage andpolarity of signals to be placed on the drive electrodes; and secondinput means for receiving a clock signal. Upon receipt of the clocksignal, the driver circuit displays the voltages selected from R+V, Rand R−V (where R and V are as defined above) on its output lines.

In another aspect, this invention provides a method for driving anelectro-optic display which displays a remnant voltage, especially anelectrophoretic display. This method comprises:

-   -   (a) applying a first driving pulse to a pixel of the display;    -   (b) measuring the remnant voltage of the pixel after the first        driving pulse; and    -   (c) applying a second driving pulse to the pixel following the        measurement of the remnant voltage, the magnitude of the second        driving pulse being controlled dependent upon the measured        remnant voltage to reduce the remnant voltage of the pixel.

This method may hereinafter for convenience be referred to as the“remnant voltage” method of the present invention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic representation of an apparatus of the presentinvention, a display which is being driven by the apparatus, andassociated apparatus, and is designed to show the overall architectureof the system;

FIG. 2 is a schematic block diagram of the controller unit shown in FIG.1 and illustrates the output signals generated by this unit;

FIG. 3 is a schematic block diagram showing the manner in which thecontroller unit shown in FIGS. 1 and 2 generates certain output signalsshown in FIG. 2;

FIGS. 4 and 5 illustrate two different sets of reference voltages whichcan be used in the display shown in FIG. 1;

FIG. 6 is a schematic representation of tradeoffs between pulse widthmodulation and voltage modulation approaches in the look-up table methodof the present invention;

FIG. 7 is a block diagram of a custom driver useful in the look-up tablemethod of the present invention;

FIG. 8 is a flow chart illustrating a program which may be run by thecontroller unit shown in FIGS. 1 and 2;

FIGS. 9 and 10 illustrate two drive schemes of the present invention;and

FIGS. 11A and 11B illustrate two parts of a third drive scheme of thepresent invention.

DETAILED DESCRIPTION

As already mentioned, the look-up table aspect of the present inventionprovides methods and controllers for driving electro-optic displayshaving a plurality of pixels, each of which is capable of displaying atleast three gray levels. The present invention may of course be appliedto electro-optic displays having a greater number of gray levels, forexample 4, 8, 16 or more.

Also as already mentioned, driving bistable electro-optic displaysrequires very different methods from those normally used to drive liquidcrystal displays (“LCD's”). In a conventional (non-cholesteric) LCD,applying a specific voltage to a pixel for a sufficient period willcause the pixel to attain a specific gray level. Furthermore, the LCmaterial is only sensitive to the magnitude of the electric field, notits polarity. In contrast, bistable electro-optic displays act asimpulse transducers, so there is no one-to-one mapping between appliedvoltage and gray state attained; the impulse (and thus the voltage)which must be applied to a pixel to achieve a given gray state varieswith the “initial” gray state of the relevant pixel. Furthermore, sincebistable electro-optic displays need to be driven in both directions(white to black, and black to white) it is necessary to specify both thepolarity and the magnitude of the impulse needed.

At this point, it is considered desirable to define certain terms whichare used herein in accordance with their conventional meaning in thedisplay art. Most of the discussion below will concentrate upon one ormore pixels of a display undergoing a single gray scale transition(i.e., a change from one gray level to another) from an “initial” stateto a “final” state. Obviously, the initial state and the final state areso designated only with regard to the single transition being consideredand in most cases the pixel with have undergone transitions prior to the“initial” state and will undergo further transitions after the “final”state. As explained below, some embodiments of the invention takeaccount not only of the initial and final states of the pixel but alsoof “prior” states, in which the pixel existed prior to achieving theinitial state. Where it is necessary to distinguish between multipleprior states, the term “first prior state” will be used to refer to thestate in which the relevant pixel existed one (non-zero) transitionprior to the initial state, the term “second prior state” will be usedto refer to the state in which the relevant pixel existed one (non-zero)transition prior to the first prior state, and so on. The term “non-zerotransition” is used to refer to a transition which effects a change ofat least one unit in gray scale; the term “zero transition” may be usedto refer to a “transition” which effects no change in gray scale of theselected pixel (although other pixels of the display may be undergoingnon-zero transitions at the same time).

As will readily be apparent to those skilled in image processing, asimple embodiment of the method of the present invention may takesaccount of only of the initial state of each pixel and the final state,and in such a case the look-up table will be two-dimensional. However,as already mentioned, some electro-optic media display a memory effectand with such media it is desirable, when generating the output signal,to take into account not only the initial state of each pixel but also(at least) the first prior state of the same pixel, in which case thelook-up table will be three-dimensional. In some cases, it may bedesirable to take into account more than one prior state of each pixel,thus resulting in a look-up table having four (if only the first andsecond prior states are taken into account) or more dimensions.

From a formal mathematical point of view, the present invention may beregarded as comprising an algorithm that, given information about theinitial, final and (optionally) prior states of an electro-optic pixel,as well as (optionally—see more detailed discussion below) informationabout the physical state of the display (e. g., temperature and totaloperating time), will produce a function V(t) which can be applied tothe pixel to effect a transition to the desired final state. From thisformal point of view, the controller of the present invention may beregarded as essentially a physical embodiment of this algorithm, thecontroller serving as an interface between a device wishing to displayinformation and an electro-optic display.

Ignoring the physical state information for the moment, the algorithmis, in accordance with the present invention, encoded in the form of alook-up table or transition matrix. This matrix will have one dimensioneach for the desired final state, and for each of the other states(initial and any prior states) are used in the calculation. The elementsof the matrix will contain a function V(t) that is to be applied to theelectro-optic medium.

The elements of the look-up table or transition matrix may have avariety of forms. In some cases, each element may comprise a singlenumber. For example, an electro-optic display may use a high precisionvoltage modulated driver circuit capable of outputting numerousdifferent voltages both above and below a reference voltage, and simplyapply the required voltage to a pixel for a standard, predeterminedperiod. In such a case, each entry in the look-up table could simplyhave the form of a signed integer specifying which voltage is to beapplied to a given pixel. In other cases, each element may comprise aseries of numbers relating to different portions of a waveform. Forexample, there are described below embodiments of the invention whichuse single- or double-prepulse waveforms, and specifying such a waveformnecessarily requires several numbers relating to different portions ofthe waveform. Also described below is an embodiment of the inventionwhich in effect applies pulse length modulation by applying apredetermined voltage to a pixel during selected ones of a plurality ofsub-scan periods during a complete scan. In such an embodiment, theelements of the transition matrix may have the form of a series of bitsspecifying whether or not the predetermined voltage is to be appliedduring each sub-scan period of the relevant transition. Finally, asdiscussed in more detail below, in some cases, such as atemperature-compensated display, it may be convenient for the elementsof the look-up table to be in the form of functions (or, in practice,more accurately coefficients of various terms in such functions).

It will be apparent that the look-up tables used in some embodiments ofthe invention may become very large. To take an extreme example,consider a process of the invention for a 256 (2⁸) gray level displayusing an algorithm that takes account of initial, final and two priorstates. The necessary four-dimensional look-up table has 2³² entries. Ifeach entry requires (say) 64 bits (8 bytes), the total size of thelook-up table would be approximately 32 Gbyte. While storing this amountof data poses no problems on a desktop computer, it may present problemsin a portable device. However, in practice the size of such largelook-up tables can be substantially reduced. In many instances, it hasbeen found that there are only a small number of types of waveformsneeded for a large number of different transitions, with, for example,the length of individual pulses of a general waveform being variedbetween different transitions. Consequently, the length of individualentries in the look-up table can be reduced by making each entrycomprises (a) a pointer to an entry in a second table specifying one ofa small number of types of waveform to be used; and (b) a small numberof parameters specifying how this general waveform should be varied forthe relevant transition.

The values for the entries in the look-up table may be determined inadvance through an empirical optimization process. Essentially, one setsa pixel to the relevant initial state, applies an impulse estimated toapproximately equal that needed to achieve the desired final state andmeasures the final state of the pixel to determine the deviation, ifany, between the actual and desired final state. The process is thenrepeated with a modified impulse until the deviation is less than apredetermined value, which may be determined by the capability of theinstrument used to measure the final state. In the case of methods whichtake into account one or more prior states of the pixel, in addition tothe initial state, it will generally be convenient to first determinethe impulse needed for a particular transition when the state of thepixel is constant in the initial state and all preceding states used indetermining the impulse, and then to “fine tune” this impulse to allowfor differing previous states.

The present method desirably provides for modification of the impulse toallow for variation in temperature and/or total operating time of thedisplay; compensation for operating time may be required because someelectro-optic media “age” and their behavior changes after extendedoperation. Such modification may be done in one of two ways. Firstly,the look-up table may be expanded by an additional dimension for eachvariable that is to be taken into account in calculating the outputsignal. Obviously, when dealing with continuous variables such astemperature and operating, it is necessary to quantize the continuousvariable in order to maintain the look-up table at a practicable finitesize. In order to find the waveform to be applied to the pixel, thecalculation means may simply choose the look-up table entry for thetable closest to the measured temperature. Alternatively, to providemore accurate temperature compensation, the calculation means may lookup the two adjacent look-up table entries on either side of the measuredcontinuous variable, and apply an appropriate interpolation algorithm toget the required entry at the measured intermediate value of thevariable. For example, assume that the matrix includes entries fortemperature in increments of 10° C. If the actual temperature of thedisplay is 25° C., the calculation would look up the entries for 20° and30° C., and use a value intermediate the two. Note that since thevariation of characteristics of electro-optic media with temperature isoften not linear, the set of temperatures for which the look-up tablestores entries may not be distributed linearly; for example, thevariation of many electro-optic media with temperature is most rapid athigh temperatures, so that at low temperatures intervals of 20° C.between look-up tables might suffice, whereas at high temperaturesintervals of 5° C. might be desirable.

An alternative method for temperature/operating time compensation is touse look-up table entries in the form of functions of the physicalvariable(s), or perhaps more accurately coefficients of standard termsin such functions. For simplicity consider the case of a display whichuses a time modulation drive scheme in which each transition is handledby applying a constant voltage (of either polarity) to each pixel for avariable length of time, so that, absent any correction forenvironmental variables, each entry in the look-up table could consistonly of a single signed number representing the duration of time forwhich the constant voltage is to be applied, and its polarity. If it isdesired to correct such a display for variations in temperature suchthat the time Tt for which the constant voltage needs to be applied fora specific transition at a temperature t is given by:

T _(t) =T ₀ +AΔt+B(Δt)²

where T₀ is the time required at some standard temperature, typicallythe mid-point of the intended operating temperature range of thedisplay, and Δt is the difference between t and the temperature at whichT₀ is measured, the entries in the look-up table can consist of thevalues of T₀, A and B for the specific transition to which a given entryrelates, and the calculation means can use these coefficients tocalculate Tt at the measured temperature. To put it more generally, thecalculation means finds the appropriate look-up table entry for therelevant initial and final states, then uses the function defined bythat entry to calculate the proper output signal having regard to theother variables to be taken into account.

The relevant temperature to be used for temperature compensationcalculations is that of the electro-optic material at the relevantpixel, and this temperature may differ significantly from ambienttemperature, especially in the case of displays intended for outdoor usewhere, for example, sunlight acting through a protective front sheet maycause the temperature of the electro-optic layer to be substantiallyhigher than ambient. Indeed, in the case of large billboard-type outdoorsigns, the temperature may vary between different pixels of the samedisplay if, for example, part of the display falls within the shadow ofan adjacent building, while the reminder is in full sunlight.Accordingly, it may be desirable to embed one or more thermocouples orother temperature sensors within or adjacent to the electro-optic layerto determine the actual temperature of this layer. In the case of largedisplays, it may also be desirable to provide for interpolation betweentemperatures sensed by a plurality of temperature sensors to estimatethe temperature of each particular pixel. Finally, in the case of largedisplays formed from a plurality of modules which can replacedindividually, the method and controller of the invention may provide fordifferent operating times for pixels in different modules.

The method and controller of the present invention may also allow forthe residence time (i.e., the period since the pixel last underwent anon-zero transition) of the specific pixel being driven. It has beenfound that, at least in some cases, the impulse necessary for a giventransition various with the residence time of a pixel in its opticalstate. Thus, it may be desirable or necessary to vary the impulseapplied for a given transition as a function of the residence time ofthe pixel in its initial optical state. In order to accomplish this, thelook-up table may optionally contain an additional dimension, which isindexed by a counter indicating the residence time of the pixel in itsinitial optical state. In addition, the controller will require anadditional storage area that contains a counter for every pixel in thedisplay. It will also require a display clock, which increments by onethe counter value stored in each pixel at a set interval. The length ofthis interval must be an integral multiple of the frame time of thedisplay, and therefore must be no less than one frame time. The size ofthis counter and the clock frequency will be determined by the length oftime over which the applied impulse will be varied, and the necessarytime resolution. For example, storing a 4-bit counter for each pixelwould allow the impulse to vary at 0.25 second intervals over a 4-secondperiod (4 seconds*4 counts/sec=16 counts=4 bits). The counter mayoptionally be reset upon the occurrence of certain events, such as thetransition of the pixel to a new state. Upon reaching its maximum value,the counter may be configured to either “roll over” to a count of zero,or to maintain its maximum value until it is reset.

The look-up table method of the present invention may of course bemodified to take account of any other physical parameter which has adetectable effect upon the impulse needed to effect any one or morespecific transitions of an electro-optic medium. For example, the methodcould be modified to incorporate corrections for ambient humidity if theelectro-optic medium is found to be sensitive to humidity.

For a bistable electro-optic medium, the look-up table will have thecharacteristic that, for any zero transition in which the initial andfinal states of the pixel are the same, the entry will be zero, or inother words, no voltage will be applied to the pixel. As a corollary, ifno pixels on the display change during a given interval, then noimpulses must be applied. This enables ultra-low power operation, aswell as ensuring that the electro-optic medium is not overdriven while astatic image is being displayed. In general, the look-up table shouldonly retain information about non-null transitions. In other words, fortwo images, I and I+1, if a given pixel is in the same state in I andI+1, then state I+1 would not be stored in the prior state table, andthat no further information will be stored until that pixel undergoes atransition.

As will readily be apparent to those skilled in modern electronictechnology, the controller of the present invention can have a varietyof physical forms. and may use any conventional data processingcomponents. For example, the present method could be practiced using ageneral purpose digital computer in conjunction with appropriateequipment (for example, one or more digital analog converters, “DAC's”)to convert the digital outputs from the computer to appropriate voltagesfor application to pixels. Alternatively, the present method could bepracticed using an application specific integrated circuit (ASIC). Inparticular, the controller of the present invention could have the formof a video card which could be inserted into a personal computer toenable the images generated by the computer to be displayed on anelectro-optic screen instead of or in addition to an existing screen,such as a LCD. Since the construction of the controller of the presentinvention is well within the level of skill in the image processing art,it is unnecessary to describe its circuitry in detail herein.

A preferred physical embodiment of the controller of the presentinvention is a timing controller integrated circuit (IC). This ICaccepts incoming image data and outputs control signals to a collectionof data and select driver IC's, in order to produce the proper voltagesat the pixels to produce the desired image. This IC may accept the imagedata through access to a memory buffer that contains the image data, orit may receive a signal intended to drive a traditional LCD panel, fromwhich it can extract the image data. It may also receive any serialsignal containing information that it requires to perform the necessaryimpulse calculations. Alternately, this timing controller can beimplemented in software, or incorporated as a part of the CPU. Thetiming controller may also have the ability to measure any externalparameters that influence the operation of the display, such astemperature.

The controller can operate as follows. The look-up table(s) are storedin memory accessible to the controller. For each pixel in turn, all ofthe necessary initial, final and (optionally) prior and physical stateinformation is supplied as inputs. The state information is then used tocompute an index into the look-up table. In the case of quantizedtemperature or other correction, the return value from this look-up willbe one voltage, or an array of voltages versus time. The controller willrepeat this process for the two bracketing temperatures in the look-uptable, then interpolate between the values. For the algorithmictemperature correction, the return value of the look-up will be one ormore parameters, which can then be inserted into an equation along withthe temperature, to determine the proper form of the drive impulse, asalready described. This procedure can be accomplished similarly for anyother system variables that require real-time modification of the driveimpulse. One or more of these system variables may be determined by, forexample, the value of a programmable resistor, or a memory location inan EPROM, which is set on the display panel at the time of constructionin order to optimize the performance of the display.

An important feature of the display controller is that, unlike mostdisplays, in most practical cases several complete scans of the displaywill be required in order to complete an image update. The series ofscans required for one image update should be considered to be anuninterruptible unit. If the display controller and image source areoperating asynchronously, then the controller must ensure that the databeing used to calculate applied impulses remains constant across allscans. This can be accomplished in one of two ways. Firstly, theincoming image data could be stored in a separate buffer by the displaycontroller (alternatively, if the display controller is accessing adisplay buffer through dual-ported memory, it could lock out access fromthe CPU). Secondly, on the first scan, the controller may store thecalculated impulses in an impulse buffer. The second option has theadvantage that the overhead for scanning the panel is only incurred onceper transition, and the data for the remaining scans can be outputdirectly from the buffer.

Optionally, imaging updating may be conducted in an asynchronous manner.Although it will, in general, take several scans to effect a completetransition between two images, individual pixels can begin transitions,or reverse transitions that have already started, in mid-frame. In orderto accomplish this, the controller must keep track of what portion ofthe total transition have been accomplished for a given pixel. If arequest is received to change the optical state of a pixel that is notcurrently in transition, then the counter for that pixel can be set tozero, and the pixel will begin transitioning on the next frame. If thepixel is actively transitioning when a new request is received, then thecontroller will apply an algorithm to determine how to reach the newstate from the current mid-transition state. For 1-bit general imageflow, one potential algorithm is simply to apply a pulse of reversepolarity, with amplitude and duration equal to the portion of theforward pulse that has already been applied.

In order to minimize the power necessary to operate a display, and tomaximize the image stability of the electro-optic medium, the displaycontroller may stop scanning the display and reduce the voltage appliedto all pixels to, or close to, zero, when there are no pixels in thedisplay that are undergoing transitions. Very advantageously, thedisplay controller may turn off the power to its associated row andcolumn drivers while the display is in such a “hold” state, thusminimizing power consumption. In this scheme, the drivers would bereactivated when the next pixel transition is requested.

FIG. 1 of the accompanying drawings shows schematically an apparatus ofthe invention in use, together with associated apparatus. The overallapparatus (generally designated 10) shown in FIG. 1 comprises an imagesource, shown as a personal computer 12 which outputs on a data line 14data representing an image. The data line 14 can be of any conventionaltype and may be a single data line or a bus; for example, the data line14 could comprise a universal serial bus (USB), serial, parallel,IEEE-1394 or other line. The data which are placed on the line 14 can bein the form of a conventional bit mapped image, for example a bit map(BMP), tagged image file format (TIF), graphics interchange format (GIF)or Jooint Photographic Experts Group (JPEG) file. Alternatively,however, the data placed on the line 14 could be in the form of signalsintended for driving a video device; for example, many computers providea video output for driving an external monitor and signals on suchoutputs may be used in the present invention. It will be apparent tothose skilled in imaging processing that the apparatus of the presentinvention described below may have to perform substantial file formatconversion and/or decoding to make use of the disparate types of inputsignals which can be used, but such conversion and/or decoding is wellwithin the level of skill in the art, and accordingly, the apparatus ofthe present invention will be described only from the point at which theimage data used as its original inputs have been converted to a formatin which they can be processed by the apparatus.

The data line 14 extends to a controller unit 16 of the presentinvention, as described in detail below. This controller unit 16generates one set of output signals on a data bus 18 and a second set ofsignals on a separate data bus 20. The data bus 18 is connected to tworow (or gate) drivers 22, while the data bus 20 is connected to aplurality of column (or source) drivers 24. (The number of columndrivers 24 is greatly reduced in FIG. 1 for ease of illustration.) Therow and column drivers control the operation of a bistable electro-opticdisplay 26.

The apparatus shown in FIG. 1 is chosen to illustrate the various unitsused, and is most suitable for a developmental, “breadboard” unit. Inactual commercial production, the controller 16 will typically be partof the same physical unit as the display 26, and the image source mayalso be part of this physical unit, as in conventional laptop computersequipped with LCD's, and in personal digital assistants. Also, thepresent invention is illustrated in FIG. 1 and will be mainly describedbelow, in conjunction with an active matrix display architecture whichhas a single common, transparent electrode (not shown in FIG. 1) on oneside of the electro-optic layer, this common electrode extending acrossall the pixels of the display. Typically, this common electrode liesbetween the electro-optic layer and the observer and forms a viewingsurface through which an observer views the display. On the opposed sideof the electro-optic layer is disposed a matrix of pixel electrodesarranged in rows and columns such that each pixel electrode is uniquelydefined by the intersection of a single row and a single column. Thus,the electric field experienced by each pixel of the electro-optic layeris controlled by varying the voltage applied to the associated pixelelectrode relative to the voltage (normally designated “Vcom”) appliedto the common front electrode. Each pixel electrode is associated withat least one transistor, typically a thin film transistor. The gates ofthe transistors in each row are connected via a single elongate rowelectrode to one of the row drivers 22. The source electrodes of thetransistors in each column are connected via a single elongate columnelectrode to one of column drivers 24. The drain electrode of eachtransistor is connected directly to the pixel electrode. It will beappreciated that the assignment of the gates to rows and the sourceelectrodes to columns is arbitrary, and could be reversed, as could theassignment of source and drain electrodes. However, the followingdescription will assume the conventional assignments.

During operation, the row drivers 22 apply voltages to the gates suchthat the transistors in one and only one row are conductive at any giventime. Simultaneously, the column drivers 24 apply predetermined voltagesto each of the column electrodes. Thus, the voltages applied to thecolumn drivers are applied to only one row of the pixel electrodes, thuswriting (or at least partially writing) one line of the desired image onthe electro-optic medium. The row driver then shifts to make thetransistors in the next row conductive, a different set of voltages areapplied to the column electrodes, and the next line of the image iswritten.

It is emphasized that the present invention is not confined to suchactive matrix displays. Once the correct waveforms for each pixel of theimage have been determined in accordance with the present invention, anyswitching scheme may be used to apply the waveforms to the pixels. Forexample, the present invention can use a so-called “direct drive”scheme, in which each pixel is provided with a separate drive line. Inprinciple, the present invention can also use a passive matrix drivescheme of the type used in some LCD's, but it should be noted that,since many bistable electro-optic media lack a threshold for switching(i.e., the media will change optical state if even a small electricfield is applied for a prolonged period), such media are unsuitable forpassive matrix driving. However, since it appears that the presentinvention will find its major application in active matrix displays, itwill be described herein primarily with reference to such displays.

The controller unit 16 (FIG. 1) has two main functions. Firstly, usingthe method of the present invention, the controller calculates atwo-dimensional matrix of impulses (or waveforms) which must be appliedto the pixels of a display to change an initial image to a final image.Secondly, the controller 16 calculates, from this matrix of impulses,all the timing signals necessary to provide the desired impulses at thepixel electrodes using the conventional drivers designed for use withLCD's to drive a bistable electro-optic display.

As shown in FIG. 2, the controller unit 16 shown in FIG. 1 has two mainsections, namely a frame buffer 16A, which buffers the data representingthe final image which the controller 16B is to write to the display 26(FIG. 1), and the controller proper, denoted 16B. The controller 16Breads data from the buffer 16A pixel by pixel and generates varioussignals on the data buses 18 and 20 as described below.

The signals shown in FIG. 2 are as follows:

-   -   D0:D5—a six-bit voltage value for a pixel (obviously, the number        of bits in this signal may vary depending upon the specific row        and column drivers used)    -   POL—pixel polarity with respect to Vcom (see below)    -   START—places a start bit into the column driver 24 to enable        loading of pixel values    -   HSYNC—horizontal synchronization signal, which latches the        column driver    -   PCLK—pixel clock, which shifts the start bit along the row        driver    -   VSYNC—vertical synchronization signal, which loads a start bit        into the row driver    -   OE—output enable signal, which latches the row driver.

Of these signals, VSYNC and OE supplied to the row drivers 22 areessentially the same as the corresponding signals supplied to the rowdrivers in a conventional active matrix LCD, since the manner ofscanning the rows in the apparatus shown in FIG. 1 is in principleidentical to the manner of scanning an LCD, although of course the exacttiming of these signals may vary depending upon the preciseelectro-optic medium used. Similarly, the START, HSYNC and PCLK signalssupplied to the column drivers are essentially the same as thecorresponding signals supplied to the column drivers in a conventionalactive matrix LCD, although their exact timing may vary depending uponthe precise electro-optic medium used. Hence, it is considered that nofurther description of these output signals in necessary.

FIG. 3 illustrates, in a highly schematic manner, the way in which thecontroller 16B shown in FIG. 2 generates the D0:D5 and POL signals. Asdescribed above, the controller 16B stores data representing the finalimage 120 (the image which it is desired to write to the display), theinitial image 122 previously written to the display, and optionally oneor more prior images 123 which were written to the display before theinitial image. The embodiment of the invention shown in FIG. 3 storestwo such prior images 123. (Obviously, the necessary data storage can bewithin the controller 16B or in an external data storage device.) Thecontroller 16B uses the data for a specific pixel (illustrated as thefirst pixel in the first row, as shown by the shading in FIG. 3) in theinitial, final and prior images 120. 122 and 123 as pointers into alook-up table 124, which provides the value of the impulse which must beapplied to the specific pixel to change the state of that pixel to thedesired gray level in the final image. The resultant output from thelook-up table 124, and the output from a frame counter 126, are suppliedto a voltage v. frame array 128, which generates the D0:D5 and POLsignals.

The controller 16B is designed for use with a TFT LCD driver that isequipped with pixel inversion circuitry, which ordinarily alternates thepolarity of neighboring pixels with respect to the top plane. Alternatepixels will be designated as even and odd, and are connected to opposingsides of the voltage ladder. Furthermore, a driver input, labeled“polarity”, serves to switch the polarity of the even and odd pixels.The driver is provided with four or more gamma voltage levels, which canbe set to determine the local slope of the voltage-level curve. Arepresentative example of a commercial integrated circuit (IC) withthese features is the Samsung KS0652 300/309 channel TFT-LCD sourcedriver. As previously discussed, the display to be driven uses a commonelectrode on one side of the electro-optic medium, the voltage appliedto this common electrode being referred to as the “top plane voltage” or“Vcom”.

In one embodiment, illustrated in FIG. 4 of the accompanying drawings,the reference voltages of the driver are arranged so that the top planevoltage is placed at one half the maximum voltage (Vmax) which thedriver can supply, i.e.

Vcom=Vmax/2

and the gamma voltages are arranged to vary linearly above and below thetop plane voltage. (FIGS. 4 and 5 are drawn assuming an odd number ofgamma voltages so that, for example, in FIG. 4 the gamma voltageVGMA(n/2+1/2) is equal to Vcom. If an even number of gamma voltages arepresent, both VGMA(n/2) and VGMA(n/2+1) are set equal to Vcom.Similarly, in FIG. 5, if an even number of gamma voltages are present,both VGMA(n/2) and VGMA(n/2+1) are set equal to the ground voltage Vss.)The pulse length necessary to achieve all needed transitions isdetermined by dividing the largest impulse needed to create the newimage by Vmax/2. This impulse can be converted into a number of framesby multiplying by the scan rate of the display. The necessary number offrames is then multiplied by two, to give an equal number of even andodd frames. These even and odd frames will correspond to whether thepolarity bit is set high or low for the frame. For each pixel in eachframe, the controller 16B must apply an algorithm which takes as itsinputs (1) whether the pixel is even or odd; (2) whether the polaritybit is high or low for the frame being considered; (3) whether thedesired impulse is positive or negative; and (4) the magnitude of thedesired impulse. The algorithm then determines whether the pixel can beaddressed with the desired polarity during that frame. If so, the properdrive voltage (impulse/pulse length) is applied to the pixel. If not,then the pixel is brought to the top plane voltage (Vmax/2) to place itin a hold state, in which no electric field is applied to the pixelduring that frame.

For example, consider two neighboring pixels in the display, an oddpixel 1 and an even pixel 2. Further, assume that when the polarity bitis high, the odd pixels will be able to access the positive drivevoltage range (i.e. above the top plane voltage), and the even pixelswill be able to access the negative voltages (i.e. below the top planevoltage). If both pixels 1 and 2 need to be driven with a positiveimpulse, then the following sequence must occur:

-   -   (a) during the positive polarity frames, pixel 1 is driven with        a positive voltage, and pixel 2 is held at the top plane        voltage; and    -   (b) during the negative polarity frames, pixel 1 is held at the        top plane voltage, while pixel 2 is driven with a positive        voltage.

Although typically frames with positive and negative polarity will beinterleaved 1:1 (i.e., will alternate with each other), but this is notnecessary; for example, all the odd frames could be grouped together,followed by all the even frames. This would result in alternate columnsof the display being driven in two separate groups.

The major advantage of this embodiment is that the common frontelectrode does not have to be switched during operation. The primarydisadvantage is that the maximum drive voltage available to theelectro-optic medium is only half of the maximum voltage of the driver,and that each line may only be driven 50% of the time. Thus. the refreshtime of such a display is four times the switching time of theelectro-optic medium under the same maximum drive voltage.

In a second embodiment of this form of the invention, the gamma voltagesof the driver are arranged as shown in FIG. 5, and the common electrodeswitches between V=0 and V=Vmax. Arranging the gamma voltages in thisway allows both even and odd pixels to be driven simultaneously in asingle direction, but requires that the common electrode be switched toaccess the opposite drive polarity. In addition, because thisarrangement is symmetric about the top plane voltage, a particular inputto the drivers will result in the same voltage being applied on eitheran odd or an even pixel. In this case, the inputs to the algorithm arethe magnitude and sign of the desired impulse, and the polarity of thetop plane. If the current common electrode setting corresponds to thesign of the desired impulse, then this value is output. If the desiredimpulse is in the opposite direction, then the pixel is set to the topplane voltage so that no electric field is applied to the pixel duringthat frame.

As in the embodiment previously described, in this embodiment thenecessary length of the drive pulse can be calculated by dividing themaximum impulse by the maximum drive voltage, and this value convertedinto frames by multiplying by the display refresh rate. Again, thenumber of frames must be doubled, to account for the fact that thedisplay can only be driven in one direction with respect to the topplane at a time.

The major advantage of this second embodiment is that the full voltageof the driver can be used, and all of the outputs can be driven at once.However, two frames are required for driving in opposed directions.Thus. the refresh time of such a display is twice the switching time ofthe electro-optic medium under the same maximum drive voltage. The majordrawback is the need to switch the common electrode, which may result inunwanted voltage artifacts in the electro-optic medium, the transistorsassociated with the pixel electrodes, or both.

In either embodiment, the gamma voltages are normally arranged on alinear ramp between the maximum voltages of the driver and the top planevoltage. Depending upon the design of the driver, it may be necessary toset one or more of the gamma voltages at the top plane value, in orderto ensure that the driver can actually produce the top plane voltage onthe output.

Reference has already been made above to the need to adapt the method ofthe present invention to the limitations of conventional driversdesigned for use with LCD's. More specifically, conventional columndrivers for LCD's, and particularly super twisted nematic (STN) LCD's(which can usually handle higher voltages than other types of columndrivers), are only capable of applying one of two voltages to a driveline at any given time, since this is all that a polarity-insensitive LCmaterial requires. In contrast, to drive polarity-sensitiveelectro-optic displays, a minimum of three driver voltage levels arenecessary. The three driver voltages required are V−, which drives apixel negative with respect to the top plane voltage, V+, which drives apixel positive with respect to the top plane voltage, and 0V withrespect to the top plane voltage, which will hold the pixel in the samedisplay state.

The method of the present invention can, however, be practiced with thistype of conventional LCD driver, provided that the controller isarranged to apply an appropriate sequence of voltages to the inputs ofone or more column drivers, and their associated row drivers, in orderto apply the necessary impulses to the pixels of an electro-opticdisplay.

There are two principal variants of this approach. In the first variant,all the impulses applied must have one of three values: +I, −I or 0,where:

+I=−(−I)=Vapp*t _(pulse)

where Vapp is the applied voltage above the top plane voltage, andt_(pulse) is the pulse length in seconds. This variant only allows thedisplay to operate in a binary (black/white) mode. In the secondvariant, the applied impulses may vary from +I to −I, but must beintegral multiples of Vapp/freq, where freq is the refresh frequency ofthe display.

This aspect of the present invention takes advantage of the fact that,as already noted, conventional LCD drivers are designed to reversepolarity at frequent intervals to avoid certain undesirable effectswhich might otherwise be produced in the display. Consequently, suchdrivers are arranged to receive from the controller a polarity orcontrol voltage, which can either be high or low. When a low controlvoltage is asserted, the output voltage on any given driver output linecan adopt one of two out of the possible three voltages required, say V1or V2, while when a high control voltage is asserted, the output voltageon any given line can adopt one of a different two of the possible threevoltages required, say V2 or V3. Thus, while only two out of the threerequired voltages can be addressed at any specific time, all threevoltages can be achieved at differing times. The three required voltageswill usually satisfy the relationship:

V2=(V3+V1)/2

and V1 may be at or near the logic ground.

In this method of the invention, the display will be scanned2*t_(pulse)*freq times. For half these scans (i.e., for t_(pulse)*freqscans), the driver will be set to output either V1 or V2, which willnormally be equal to −V and Vcom, respectively. Thus, during thesescans, the pixels are either driven negative, or held in the samedisplay state. For the other half of the scans, the driver will beswitched to output either V2 or V3, which will normally be at Vcom and+V respectively. In these scans, the pixels are driven positive or heldin the same display state. Table 1 below illustrates how these optionscan be combined to produce a drive in either direction or a hold state;the correlation of positive driving with approach to a dark state andnegative driving with approach to a light state is of course a functionof the specific electro-optic medium used.

TABLE 1 Driver outputs Desired Drive V1 - V2 V2 - V3 positive (drivedark) V2 V3 negative (drive white) V1 V2 hold V2 V2

There are several different ways to arrange the two portions of thedrive scheme (i.e., the two different types of scans or “frames”). Forexample, the two types of frames could alternate. If this is done at ahigh refresh rate, then the electro-optic medium will appear to besimultaneously lightening and darkening, when in fact it is being drivenin opposed direction in alternate frames. Alternatively, all of theframes of one type could occur before any of the frames of the secondtype; this would result in a two-step drive appearance. Otherarrangements are of course possible; for example two or more frames ofone type followed by two or more of the opposed type. Additionally, ifthere are no pixels that need to be driven in one of the two directions,then the frames of that polarity can be dropped, reducing the drive timeby 50%.

While this first variant can only produce binary images, the secondvariant can render images with multiple gray scale levels. This isaccomplished by combining the drive scheme described above withmodulation of the pulse widths for different pixels. In this case, thedisplay is again scanned 2*t_(pulse)*freq times, but the driving voltageis only applied to any particular pixel during enough of these scans toensure that the desired impulse for that particular pixel is achieved.For example, for each pixel, the total applied impulse could berecorded, and when the pixel reached its desired impulse, the pixelcould be held at the top plane voltage for all subsequent scans. Forpixels that need to be driven for less than the total scanning time, thedriving portion of this time (i.e., the portion of the time during whichan impulse is applied to change the display state of the pixel, asopposed to the holding portion during which the applied voltage simplymaintains the display state of the pixel) may be distributed in avariety of ways within the total time. For example, all driving portionscould be set to start at the beginning of the total time, or all drivingportions could instead be timed to complete at the end of the totaltime. As in the first variant, if at any time in the second variant nofurther impulses of a particular polarity need to be applied to anypixel, then the scans applying pulses of that polarity can beeliminated. This may mean that the entire pulse is shortened, forexample, if the maximum impulse to be applied in both the positive andnegative directions is less than the maximum allowable impulse.

To take a highly simplified case for purposes of illustration, considerthe application of the gray scale scheme described above to a displayhaving four gray levels, namely black (level 0), dark gray (level 1),light gray (level 2) and white (level 3). One possible drive scheme forsuch a display is summarized in Table 2 below.

TABLE 2 Frame No. 1 2 3 4 5 6 Parity Transition Odd Even Odd Even OddEven 0-3 + 0 + 0 + 0 0-2 + 0 + 0 0 0 0-1 + 0 0 0 0 0 0-0 0 0 0 0 0 0 3-00 − 0 − 0 − 2-0 0 − 0 − 0 0 1-0 0 − 0 0 0 0

For ease of illustration, this drive scheme is assumed to use only sixframes although in practice a greater number would typically beemployed. These frames are alternately odd and even. White-goingtransitions (i.e., transitions in which the gray level is increased) aredriven only on the odd frames, while black-going transitions (i.e.,transitions in which the gray level is decreased) are driven only on theeven frames. On any frame when a pixel is not being driven, it is heldat the same voltage as the common front electrode, as indicated by “0”in Table 2. For the 0-3 (black-white) transition, a white-going impulseis applied (i.e., the pixel electrode is held at a voltage relative tothe common front electrode which tends to increase the gray level of thepixel) in each of the odd frames, Frames 1, 3 and 5. For a 0-2 (black tolight gray) transition, on the other hand, a white-going impulse isapplied only in Frames 1 and 3, and no impulse is applied in Frame 5;this is of course arbitrary, and, for example, a white-going impulsecould be applied in Frames 1 and 5 and no impulse applied in Frame 3.For a 0-1 (black to dark gray) transition, a white-going impulse isapplied only in Frame 1, and no impulse is applied in Frames 3 and 5;again, this is arbitrary, and, for example, a white-going impulse couldbe applied in Frame 3 and no impulse applied in Frames 1 and 5.

The black-going transitions are handled in a manner exactly similar tothe corresponding white-going transitions except that the black-goingimpulses are applied only in the even frames of the drive scheme. It isbelieved that those skilled in driving electro-optic displays willreadily be able to understand the manner in which the transitions notshown in Table 2 are handled from the foregoing description.

The sets of impulses described above can either be stand-alonetransitions between two images (as in general image flow), or they maybe part of a sequence of impulses designed to accomplish an imagetransition (as in a slide-show waveform).

Although emphasis has been laid above on methods of the presentinvention which permit the use of conventional drivers designed for usewith LCD's, the present invention can make use of custom drivers, and adriver which is intended to enable accurate control of gray states in anelectro-optic display, while achieving rapid writing of the display willnow be described with reference to FIGS. 6 and 7.

As already discussed, to first order, many electro-optic media respondto a voltage impulse, which can be expressed as V times t (or moregenerally, by the integral of V with respect to t) where V is thevoltage applied to a pixel and t is the time over which the voltage isapplied. Thus, gray states can be obtained by modulating the length ofthe voltage pulse applied to the display, or by modulating the appliedvoltage, or by a combination of these two.

In the case of pulse width modulation in an active matrix display, theattainable pulse width resolution is simply the inverse of the refreshrate of the display. In other words, for a display with a 100 Hz refreshrate, the pulse length can be subdivided into 10 ms intervals. This isbecause each pixel in the display is only addressed once per scan, whenthe select line for the pixels in that row are activated. For the restof the time, the voltage on the pixel may be maintained by a storagecapacitor, as described in the aforementioned WO 01/07961. As theresponse speed of the electro-optic medium becomes faster, the slope ofthe reflectivity versus time curve becomes steeper and steeper. Thus, tomaintain the same gray scale resolution, the refresh rate of the displaymust increase accordingly. Increasing the refresh rate results in higherpower consumption, and eventually becomes impractical as the transistorsand drivers are expected to charge the pixel and line capacitance in ashorter and shorter time.

On the other hand, in a voltage modulated display, the impulseresolution is simply determined by the number of voltage steps, and isindependent of the speed of the electro-optic medium. The effectiveresolution can be increased by imposing a nonlinear spacing of thevoltage steps, concentrating them where the voltage/reflectivityresponse of the electro-optic medium is steepest.

FIG. 6 of the accompanying drawings is a schematic representation of thetradeoffs between the pulse width modulation (PWM) and voltagemodulation (VM) approaches. The horizontal axis represents pulse length,while the vertical axis represents voltage. The reflectivity of theparticle-based electrophoretic display as a function of these twoparameters is represented as a contour plot, with the bands and spacesrepresenting differences of 1 L* in the reflected luminance of thedisplay, where L* has the usual ICE definition:

L*=116(R/R ₀)^(1/3)−16

where R is the reflectance and R₀ is a standard reflectance value. (Ithas been found empirically that a difference in luminance of 1 L* isjust noticeable to an average subject in dual stimulus experiments.)This particular particle-based electrophoretic medium used in theexperiments summarized in FIG. 6 had a response time of 200 ms at themaximum voltage (16 V) shown in the Figure.

The effects of pulse width modulation alone can be determined bytraversing the plot horizontally along the top, while the effects ofvoltage modulation alone are seen by examining the right vertical edge.From this plot, it is clear that, if a display using this particularmedium were driven at a refresh rate of 100 Hz in a pulse widthmodulation (PWM) mode, it would not be possible to obtain a reflectivitywithin ±1 L* in the middle gray region, where the contours are steepest.In voltage modulation (VM) mode, achieving a reflectivity within ±1 L*would require 128 equally spaced voltage levels, while running at aframe rate as low as 5 Hz (assuming, of course, that the voltage holdingcapability of the pixel, provided by a capacitor, is high enough). Inaddition, these two approaches can be combined to achieve the sameaccuracy with fewer voltage levels. To further reduce the requirednumber of voltage levels, they could be concentrated in the steep middleportion of the curves shown in FIG. 6 but made sparse in the outerregions. This could be accomplished with a small number of input gammavoltages. To further reduce the required number of voltage levels, theycould be concentrated at advantageous values. For example, very smallvoltages are not useful for achieving transitions if application of sucha small voltage over the allotted address time is not sufficient to makeany of the desired gray state transitions. Choosing a distribution ofvoltages that excludes such small voltages allows the allowed voltagesto be more advantageously placed.

Since bistable electro-optic displays are sensitive to the polarity ofthe applied electric field, as noted above, it is not desirable toreverse the polarity of the drive voltage on successive frames (images),as is usually done with LCD's, and frame, pixel and line inversion areunnecessary, and indeed counterproductive. For example, LCD drivers withpixel inversion deliver voltages of alternating polarity in alternateframes. Thus, it is only possible to deliver an impulse of the properpolarity in one half of the frames. This is not a problem in an LCD,where the liquid crystal material in not sensitive to polarity, but in abistable electro-optic display it doubles the time required to addressthe electro-optic medium.

Similarly, because bistable electro-optic displays are impulsetransducers and not voltage transducers, the displays integrate voltageerrors over time, which can result in large deviations of the pixels ofthe display from their desired optical states. This makes it importantto use drivers with high voltage accuracy, and a tolerance of ±3 mV orless is recommended.

To enable a driver to address a monochrome XGA (1024×768) display panelat a 75 Hz refresh rate, a maximum pixel clock rate of 60 MHz isrequired; achieving this clock rate is within the state of the art.

As already mentioned, one of the primary virtues of particle-basedelectrophoretic and other similar bistable electro-optic displays istheir image stability, and the consequent opportunity to run the displayat very low power consumption. To take maximum advantage of thisopportunity, power to the driver should be disabled when the image isnot changing. Accordingly, the driver should be designed to power downin a controlled manner, without creating any spurious voltages on theoutput lines. Because entering and leaving such a “sleep” mode will be acommon occurrence, the power-down and power-up sequences should be asrapid as possible, and should have minimal effects on the lifetime ofthe driver.

In addition, there should be an input pin that brings all of the driveroutput pins to Vcom, which will hold all of the pixels at their currentoptical state without powering down the driver.

The drivers of the present invention are useful, inter alia, for drivingmedium to high resolution, high information content portable displays,for example a 7 inch (178 mm) diagonal XGA monochrome display. Tominimize the number of integrated circuits required in such highresolution panels, it is desirable to use drivers with a high number(for example, 324) of outputs per package. It is also desirable that thedriver have an option to run in one or more other modes with fewer ofits outputs enabled. The preferred method for attaching the integratedcircuits to the display panel is tape carrier package (TCP), so it isdesirable to arrange the sizing and spacing of the driver outputs tofacilitate use of this method.

The present drivers will typically be used to drive small to mediumactive matrix panels at around 30 V. Accordingly, the drivers should becapable of driving capacitative loads of approximately 100 pF.

A block diagram of a preferred driver (generally designated 200) of theinvention is given in FIG. 7 of the accompanying drawings. This driver200 comprises a shift register 202, a data register 204, a data latch206, a digital to analogue converter (DAC) 208 and an output buffer 210.This driver differs from those typically used to drive LCD's in that itprovides for a polarity bit associated with each pixel of the display,and for generating an output above or below the top plane voltagecontrolled by the relevant polarity bit.

The signal descriptions for this preferred driver are given in thefollowing Table 3:

TABLE 3 Symbol Pin Name Description VDD Logic power supply 2.7-3.6 VAVDD Driver power supply 10-30 V VSS Ground 0 V Y1-Y324 Driver outputs,fed to the D/A converted 64 level column electrodes of the analogoutputs display D0(0:5) Display data input, odd 6 bit gray scale datafor odd dots dots, D0: 0 = least significant bit (LSB) D1(0:5) Displaydata input, even 6 bit gray scale data for dots even dots, D1: 0 = LSBD0POL Odd dot polarity control Determines which set of input gammavoltages current odd dot will reference. D0POL = 1: odd dot willreference VGAM6-11 D0POL = 0: odd dot will reference VGAM1-6 D1POL Evendot polarity control Determines which set of input gamma voltagescurrent even dot will reference. D1POL = 1: odd dot will referenceVGAM6-11 D1POL = 0: odd dot will reference VGAM1-6 SHL Shift directioncontrol Controls shift direction in 162 input bit shift register SHL =H: DIO1 input, Y1−>Y324 SHL = L: DIO1 output, Y324−>Y1 DIO1 Start pulseinput/output SHL = H: Used as the start pulse input pin SHL = L: Used asthe start pulse output pin DIO2 Start pulse input/output for SHL = H:Used as the start 256 lines pulse output pin for 256 lines active SHL =L: Used as the start pulse input pin for 256 lines, tie low if not usedDIO3 Start pulse input/output for SHL = H: Used as the start 260 linespulse output pin for 260 lines active SHL = L: Used as the start pulseinput pin for 260 lines, tie low if not used DIO4 Start pulseinput/output for SHL = H: Used as the start 300 lines pulse output pinfor 300 lines active SHL = L: Used as the start pulse input pin for 300lines, tie low if not used DIO5 Start pulse input/output for SHL = H:Used as the start 304 lines pulse output pin for 304 lines active SHL =L: Used as the start pulse input pin for 304 lines, tie low if not usedDIO6 Start pulse input/output for SHL = H: Used as the start 320 linespulse output pin for 320 lines active SHL = L: Used as the start pulseinput pin for 320 lines, tie low if not used DIO7 Start pulseinput/output for SHL = H: Used as the start 324 lines pulse output pinfor 324 lines active SHL = L: Used as the start pulse input pin for 324lines, tie low if not used CLK1 Shift clock input Two 6 bit gray valuesand two polarity control values for two display dots are loaded at everyrising edge CLK2 Latch input Latches the contents of the data registeron a rising edge and transfers latched values to the D/A converterblock. BL Blanking input (this does Sets all outputs to VGAM6 notactually blank the level BL = H: All outputs set bistable display, butto VGAM6 BL = L: All simply stops the driver outputs reflect D/A valueswriting to ths display, thus allowing the image already written toremain) VGAM1-6 Lower gamma reference Determine grayscale voltagevoltages outputs through resistive DAC system VGAM6-11 Upper gammareference Determine grayscale voltage voltages outputs through resistiveDAC system

The driver 200 operates in the following manner. First, a start pulse isprovided by setting (say) DIO1 high to reset the shift register 202 to astarting location. (As will readily be apparent to those skilled indisplay driver technology, the various DIOx inputs to the shift registerare provided to enable the driver to be used with displays havingvarying numbers of columns, and only one of these inputs is used withany given display, the others being tied permanently low.) The shiftregister now operates in the conventional manner used in LCD's; at eachpulse of CLK1, one and only one of the 162 outputs of the shift register202 goes high, the others being held low, and the high output beingshifted one place at each pulse of CLK1. As schematically indicated inFIG. 7, each of the 162 outputs of the shift register 202 is connectedto two inputs of data register 204, one odd input and one even input.

The display controller (cf. FIG. 2) provides two six-bit impulse valuesD0(0:5) and D1(0:5) and two single-bit polarity signals D0POL and D1POLon the inputs of the data register 204. At the rising edge of each clockpulse CLK1, two seven-bit numbers (D0POL+D0(0:5) and D1POL+D1(0:5)) arewritten into registers in data register 204 associated with the selected(high) output of shift register 202. Thus, after 162 clock pulses CLK1,324 seven-bit numbers (corresponding to the impulse values for onecomplete line of the display for one frame) have been written into the324 registers present in data register 204.

At the rising edge of each clock pulse CLK2, these 324 seven-bit numbersare transferred from the data register 204 to the data latch 206. Thenumbers thus placed in the data latch 206 are read by the DAC 208 and,in conventional fashion, corresponding analogue values are placed on theoutputs of the DAC 208 and fed, via the buffer 210 to the columnelectrodes of the display, where they are applied to pixel electrodes ofone row selected in conventional fashion by a row driver (not shown). Itshould be noted, however, that the polarity of each column electrodewith respect to Vcom is controlled by the polarity bit D0POL or D1POLwritten into the data latch 206 and thus these polarities do notalternate between adjacent column electrodes in the conventional mannerused in LCD's.

FIG. 8 is a flow chart illustrating a program which may be run by thecontroller unit shown in FIGS. 1 and 2. This program (generallydesignated 300) is intended for use with a look-up table method of theinvention (described in more detail below) in which all pixels of adisplay are erased and then re-addressed each time an image is writtenor refreshed.

The program begins with a “powering on” step 302 in which the controlleris initialized, typically as a result of user input, for example a userpushing the power button of a personal digital assistant (PDA). The step302 could also be triggered by, for example, the opening of the case ofa PDA (this opening being detected either by a mechanical sensor or by aphotodetector), by the removal of a stylus from its rest in a PDA, bydetection of motion when a user lifts a PDA, or by a proximity detectorwhich detects when a user's hand approaches a PDA.

The next step 304 is a “reset” step in which all the pixels of thedisplay are driven alternately to their black and white states. It hasbeen found that, in at least some electro-optic media, such “flashing”of the pixels is necessary to ensure accurate gray states during thesubsequent writing of an image on the display. It has also been foundthat typically at least 5 flashes (counting each successive black andwhite state as one flash) are required, and in some cases more. Thegreater the number of flashes, the more time and energy that this stepconsumes, and thus the longer the time that must elapse before the usercan see a desired image upon the display. Accordingly, it is desirablethat the number of flashes be kept as small as possible consistent withaccurate rendering of gray states in the image subsequently written. Atthe conclusion the reset step 304, all the pixels of the display are inthe same black or white state.

The next step 306 is a writing or “sending out image” step in which thecontroller 16 sends out signals to the row and column drivers 22 and 24respectively (FIGS. 1 and 2) in the manner already described, thuswriting a desired image on the display. Since the display is bistable,once the image has been written, it does not need to be rewrittenimmediately, and thus after writing the image, the controller can causethe row and column drivers to cease writing to the display, typically bysetting a blanking signal (such as setting signal BL in FIG. 7 high).

The controller now enters a decision loop formed by steps 308, 310 and312. In step 308, the controller 16 checks whether the computer 12(FIG. 1) requires display of a new image. If so, the controllerproceeds, in an erase step 314 to erase the image written to the displayat step 306, thus essentially returning the display to the state reachedat the end of reset step 304. From erase step 314, the controllerreturns to step 304, resets as previously described, and proceeds towrite the new image.

If at step 308 no new image needs to be written to the display, thecontroller proceeds to a step 310, at which it determines when the imagehas remained on the display for more than a predetermined period. As iswell known to those skilled in display technology, images written onbistable media do not persist indefinitely, and the images graduallyfade (i.e., lose contrast). Furthermore, in some types of electro-opticmedium, especially electrophoretic media, there is often a trade-offbetween writing speed of the medium and bistability, in that media whichare bistable for hours or days have substantially longer writing timesthan media which are only bistable for seconds or minutes. Accordingly,although it is not necessary to rewrite the electro-optic mediumcontinuously, as in the case of LCD's, to provide an image with goodcontrast, it may be desirable to refresh the image at intervals of (say)a few minutes. Thus, at step 310 the controller determines whether thetime which has elapsed since the image was written at step 306 exceedssome predetermined refresh interval, and if so the controller proceedsto erase step 314 and then to reset step 304, resets the display aspreviously described, and proceeds to rewrite the same image to thedisplay.

(The program shown in FIG. 8 may be modified to make use of both localand global rewriting, as discussed in more detail below. If so, step 310may be modified to decide whether local or global rewriting is required.If, in this modified program, at step 310 the program determines thatthe predetermined time has not expired, no action is taken. If, however,the predetermined time has expired, step 310 does not immediately invokeerasure and rewriting of the image; instead step 310 simply sets a flag(in the normal computer's sense of that term) indicating that the nextimage update should be effected globally rather than locally. The nexttime the program reaches step 306, the flag is checked; if the flag isset, the image is rewritten globally and then the flag is cleared, butif the flag is not set, only local rewriting of the image is effected.)

If at step 310 it is determined that the refresh interval has not beenexceeded, the controller proceeds to a step 312, where it determineswhether it is time to shut down the display and/or the image source. Inorder to conserve energy in a portable apparatus, the controller willnot allow a single image to be refreshed indefinitely, and terminatesthe program shown in FIG. 8 after a prolonged period of inactivity.Accordingly, at step 310 the controller determines whether apredetermined “shut-down” period (greater than the refresh intervalmentioned above) has elapsed since a new image (rather than a refresh ofa previous image) was written to the display, and if so the programterminates, as indicated at 314. Step 314 may include powering down theimage source. Naturally, the user still has access to a slowly-fadingimage on the display after such program termination. If the shut-downperiod has not been exceeded, the controller proceeds from step 312 backto step 308.

Various possible waveforms for carrying out the look-up table method ofthe present invention will now be described, though by way of exampleonly. However, first some general considerations regarding waveforms tobe used in the present invention will be discussed.

Waveforms for bistable displays that exhibit the aforementioned memoryeffect can be grouped into two major classes, namely compensated anduncompensated. In a compensated waveform, all of the pulses areprecisely adjusted to account for any memory effect in the pixel. Forexample, a pixel undergoing a series of transitions through gray scalelevels 1-3-4-2 might receive a slightly different impulse for the 4-2transition than a pixel that undergoes a transition series 1-2-4-2. Suchimpulse compensation could occur by adjusting the pulse length, thevoltage, or by otherwise changing the V(t) profile of the pulses. In anuncompensated waveform, no attempt is made to account for any priorstate information (other than the initial state). In an uncompensatedwaveform, all pixels undergoing the 4-2 transition would receiveprecisely the same pulse. For an uncompensated waveform to worksuccessfully, one of two criteria must be met. Either the electro-opticmaterial must not exhibit a memory effect in its switching behavior, oreach transition must effectively eliminate any memory effect on thepixel.

In general, uncompensated waveforms are best suited for use with systemscapable of only coarse impulse resolution. Examples would be a displaywith tri-level drivers, or a display capable of only 2-3 bits of voltagemodulation. A compensated waveform requires fine impulse adjustments,which are not possible with these systems. Obviously, while acoarse-impulse system is preferably restricted to uncompensatedwaveforms, a system with fine impulse adjustment can implement eithertype of waveform.

The simplest uncompensated waveform is 1-bit general image flow (1-bitGIF). In 1-bit GIF, the display transitions smoothly from one pureblack-and-white image to the next. The transition rule for this sequencecan be stated simply: If a pixel is switching from white to black, thenapply an impulse I. If it is switching from black to white, apply theimpulse of the opposite polarity, −I. If a pixel remains in the samestate, then no impulse is applied to that pixel. As previously stated,the mapping of the impulse polarity to the voltage polarity of thesystem will depend upon the response function of the material.

Another uncompensated waveform that is capable of producing grayscaleimages is the uncompensated n-prepulse slide show (n-PP SS). Theuncompensated slide show waveform has three basic sections. First, thepixels are erased to a uniform optical state, typically either white orblack. Next, the pixels are driven back and forth between two opticalstates, again typically white and black. Finally, the pixel is addressedto a new optical state, which may be one of several gray states. Thefinal (or writing) pulse is referred to as the addressing pulse, and theother pulses (the first (or erasing) pulse and the intervening (orblanking) pulses) are collectively referred to as prepulses. A waveformof this type will be described below with reference to FIGS. 9 and 10.

Prepulse slide show waveforms can be divided into two basic forms, thosewith an odd number of prepulses, and those with an even number ofprepulses. For the odd-prepulse case, the erasing pulse may be equal inimpulse and opposite in polarity to the immediately previous writingpulse (again, see FIG. 9 and discussion thereof below). In other words,if the pixel is written to gray from black, the erasing pulse will takethe pixel back to the black state. In the even-prepulse case, theerasing pulse should be of the same polarity as the previous writingpulse, and the sum of the impulses of the previous writing pulse and theerasing pulse should be equal to the impulse necessary to fullytransition from black to white. In other words, if a pixel is writtenfrom black in the even-prepulse case, then it must be erased to white.

After the erasing pulse, the waveform includes either zero or an evennumber of blanking pulses. These blanking pulses are typically pulses ofequal impulse and opposite polarity, arranged so that the first pulse isof opposite polarity to the erasing pulse. These pulses will generallybe equal in impulse to a full black-white pulse, but this is notnecessarily the case. It is also only necessary that pairs of pulseshave equal and opposite impulses it is possible that there may be pairsof widely varying impulses chained together, i.e. +I, −I, +0.1I, −0.1I,+4I, −4I.

The last pulse to be applied is the writing pulse. The impulse of thispulse is chosen based only upon the desired optical state (not upon thecurrent state, or any prior state). In general, but not necessarily, thepulse will increase or decrease monotonically with gray state value.Since this waveform is specifically designed for use with coarse impulsesystems, the choice of the writing pulse will generally involve mappinga set of desired gray states onto a small number of possible impulsechoices, e.g. 4 gray states onto 9 possible applied impulses.

Examination of either the even or odd form of the uncompensatedn-prepulse slide show waveform will reveal that the writing pulse alwaysbegins from the same direction, i.e. either from black or from white.This is an important feature of this waveform. Since the principle ofthe uncompensated waveform is that the pulse length can not becompensated accurately to ensure that pixels reach the same opticalstate, one cannot to expect to reach an identical optical state whenapproaching from opposite extreme optical states (black or white).Accordingly, there are two possible polarities for either of theseforms, which can be labeled “from black” and “from white.”

One major shortcoming of this type of waveform is that it haslarge-amplitude optical flashes between images. This can be improved byshifting the update sequence by one superframe time for half of thepixels, and interleaving the pixels at high resolution, as discussedbelow with reference to FIGS. 9 and 10. Possible patterns include everyother row, every other column, or a checkerboard pattern. Note, thisdoes not mean using the opposite polarity, i.e. “from black” vs “fromwhite”, since this would result in non-matching gray scales onneighboring pixels. Instead, this can be accomplished by delaying thestart of the update by one “superframe” (a grouping of frames equivalentto the maximum length of a black-white update) for half of the pixels(i.e. the first set of pixels completes the erase pulse, then the secondset of pixels begin the erase pulse as the first set of pixels begin thefirst blanking pulse). This will require the addition of one superframefor the total update time, to allow for this synchronization.

It might at first appear that the ideal method of the present inventionwould be so-called “general grayscale image flow” in which thecontroller arranges each writing of an image so that each pixeltransitions directly from its initial gray level to its final graylevel. In practice, however, general grayscale image flow suffers froman accumulation of errors problem. The impulse applied in any givengrayscale transition will necessarily differ from that theoreticallynecessary because of facts such as unavoidable variations in thevoltages output by drivers, manufacturing variations in the thickness ofthe electro-optic medium, etc. Suppose that the average error on eachtransition, expressed in terms of the difference between the theoreticaland the actual reflectance of the display is ±0.2 L*. After 100successive transitions, the pixels will display an average deviationfrom their expected state of 2 L*; such deviations are apparent to theaverage observer on certain types of images. To avoid this problem, itmay be desirable to arrange the drive scheme used in the presentinvention so that any given pixel can only undergo a predeterminedmaximum number of gray scale transitions before passing through oneextreme optical state (black or white). These extreme optical states actas “rails” in that after a particular impulse has been applied to anelectro-optic medium, the medium cannot become any blacker or whiter.Thus, the next transition away from the extreme optical state can startfrom an accurately known optical state, in effect canceling out anypreviously accumulated errors. Various techniques for minimizing theoptical effects of such passage of pixels through extreme optical statesare discussed below.

A first, simple drive scheme useful in the present invention will now bedescribed with reference to a simple two-bit gray scale system havingblack (level 0), dark gray (level 1), light gray (level 2) and white(level 3) optical states, transitions being effected using a pulse widthmodulation technique, and a look-up table for transitions as set out inTable 4 below.

TABLE 4 Transition Impulse Transition Impulse 0-0 0 0-0 0 0-1  n 1-0  −n0-2 2n 2-0 −2n 0-3 3n 3-0 −3nwhere n is a number dependent upon the specific display, and −nindicates a pulse having the same length as a pulse n but of oppositepolarity. It will further be assumed that at the end of the reset pulse304 in FIG. 8 all the pixels of the display are black (level 0). Since,as described below, all transitions take place through an interveningblack state, the only transitions effected are those to or from thisgray state. Thus, the size of the necessary look-up table issignificantly reduced, and obviously the factor by which look-up tablesize is thus reduced increases with the number of gray levels of thedisplay.

FIG. 9 shows the transitions of one pixel associated with the drivescheme of FIG. 8. At the beginning of the reset step 304, the pixel isin some arbitrary gray state. During the reset step 304, the pixel isdriven alternately to three black states and two intervening whitestates, ending in its black state. The pixel is then, at 306, writtenwith the appropriate gray level for a first image, assumed to belevel 1. The pixel remains at this level for some time during which thesame image is displayed; the length of this display period is greatlyreduced in FIG. 9 for ease of illustration. At some point, a new imageneeds to be written, and at this point, the pixel is returned to black(level 0) in erase step 308, and is then subjected, in a second resetstep designated 304′, to six reset pulses, alternately white and black,so that at the end of this reset step 304′, the pixel has returned to ablack state. Finally, in a second writing step designated 306′, thepixel is written with the appropriate gray level for a second image,assumed to be level 2.

Numerous variations of the drive scheme shown in FIG. 9 are of coursepossible. One useful variation is shown in FIG. 10. The steps 304, 306and 308 shown in FIG. 10 are identical to those shown in FIG. 9.However, in step 304′, five reset pulses are used (obviously a differentodd number of pulses could also be used), so that at the end of step304′, the pixel is in a white state (level 3), and in the second writingstep 306′, writing of the pixel is effected from this white state ratherthan the black state as in FIG. 9. Successive images are then writtenalternately from black and white states of the pixel.

In a further variation of the drive schemes shown in FIGS. 9 and 10,erase step 308 is effected to as to drive the pixel white (level 3)rather than black. An even number of reset pulses are then applied tothat the pixel ends the reset step in a white state, and the secondimage is written from this white state. As with the drive scheme shownin FIG. 10, in this scheme successive images are written alternatelyfrom black and white states of the pixel.

It will be appreciated that in all the foregoing schemes, the number andduration of the reset pulses can be varied depending upon thecharacteristics of the electro-optic medium used. Similarly, voltagemodulation rather than pulse width modulation may be used to vary theimpulse applied to the pixel.

The black and white flashes which appear on the display during the resetsteps of the drive schemes described above are of course visible to theuser and may be objectionable to many users. To lessen the visual effectof such reset steps, it is convenient to divide the pixels of thedisplay into two (or more) groups and to apply different types of resetpulses to the different groups. More specifically, if it necessary touse reset pulses which drive any given pixel alternately black andwhite, it is convenient to divide the pixels into at least two groupsand to arrange the drive scheme so that one group of pixels are drivenwhite at the same time that another group are driven black. Provided thespatial distribution of the two groups is chosen carefully and thepixels are sufficiently small, the user will experience the reset stepas an interval of gray on the display (with perhaps some slightflicker), and such a gray interval is typically less objectionable thana series of black and white flashes.

For example, in one form of such a “two group reset” step, the pixel inodd-numbered columns may be assigned to one “odd” group and the pixelsin the even-numbered columns to the second “even” group. The odd pixelscould then make use of the drive scheme shown in FIG. 9, while the evenpixels could make use of a variant of this drive scheme in which, duringthe erase step, the pixels are driven to a white rather a black state.Both groups of pixels would then be subjected to an even number of resetpulses during reset step 304′, so that the reset pulses for the twogroups are essentially 180° out of phase, and the display appears graythroughout this reset step. Finally, during the writing of the secondimage at step 306′, the odd pixels are driven from black to their finalstate, while the even pixels are driven from white to their final state.In order to ensure that every pixel is reset in the same manner over thelong term (and thus that the manner of resetting does not introduce anyartifacts on to the display), it is advantageous for the controller toswitch the drive schemes between successive images, so that as a seriesof new images are written to the display, each pixel is written to itsfinal state alternately from black and white states.

Obviously, a similar scheme can be used in which the pixels inodd-numbered rows form the first group and the pixels in even-numberedrows the second group. In a further similar drive scheme, the firstgroup comprises pixels in odd-numbered columns and odd-numbered rows,and even-numbered columns and even-numbered rows, while the second groupcomprises in odd-numbered columns and even-numbered rows, andeven-numbered columns and odd-numbered rows, so that the two groups aredisposed in a checkerboard fashion.

Instead of or in addition to dividing the pixels into two groups andarranging for the reset pulses in one group to be 180° out of phase withthose of the other group, the pixels may be divided into groups whichuse different reset steps differing in number and frequency of pulses.For example, one group could use the six pulse reset sequence shown inFIG. 9, while the second could use a similar sequence having twelvepulses of twice the frequency. In a more elaborate scheme, the pixelscould be divided into four groups, with the first and second groupsusing the six pulse scheme but 180° out of phase with each other, whilethe third and fourth groups use the twelve pulse scheme but 180° out ofphase with each other.

Another scheme for reducing the objectionable effects of reset stepswill now be described with reference to FIGS. 11A and 11B. In thisscheme, the pixels are again divided into two groups, with the first(even) group following the drive scheme shown in FIG. 11A and the second(odd) group following the drive scheme shown in FIG. 11B. Also in thisscheme, all the gray levels intermediate black and white are dividedinto a first group of contiguous dark gray levels adjacent the blacklevel, and a second group of contiguous light gray levels adjacent thewhite level, this division being the same for both groups of pixels.Desirably but not essentially, there are the same number of gray levelsin these two groups; if there are an odd number of gray levels, thecentral level may be arbitrarily assigned to either group. For ease ofillustration, FIGS. 11A and 11B show this drive scheme applied to aneight-level gray scale display, the levels being designated 0 (black) to7 (white); gray levels 1, 2 and 3 are dark gray levels and gray levels4, 5 and 6 are light gray levels.

In the drive scheme of FIGS. 11A and 11B, gray to gray transitions arehandled according to the following rules:

-   -   (a) in the first, even group of pixels, in a transition to a        dark gray level, the last pulse applied is always a white-going        pulse (i.e., a pulse having a polarity which tends to drive the        pixel from its black state to its white state), whereas in a        transition to a light gray level, the last pulse applied is        always a black-going pulse;    -   (b) in the second, odd group of pixels, in a transition to a        dark gray level, the last pulse applied is always a black-going        pulse, whereas in a transition to a light gray level, the last        pulse applied is always a white-going pulse;    -   (c) in all cases, a black-going pulse may only succeed a        white-going pulse after a white state has been attained, and a        white-going pulse may only succeed a black-going pulse after a        black state has been attained; and    -   (d) even pixels may not be driven from a dark gray level to        black by a single black-going pulse nor odd pixels from a light        gray level to white using a single white-going pulse.        (Obviously, in both cases, a white state can only be achieved        using a final white-going pulse and a black state can only be        achieved using a final black-going pulse.)

The application of these rules allows each gray to gray transition to beeffected using a maximum of three successive pulses. For example, FIG.11A shows an even pixel undergoing a transition from black (level 0) togray level 1. This is achieved with a single white-going pulse (shown ofcourse with a positive gradient in FIG. 11A) designated 1102. Next, thepixel is driven to gray level 3. Since gray level 3 is a dark graylevel, according to rule (a) it must be reached by a white-going pulse,and the level 1/level 3 transition can thus be handled by a singlewhite-going pulse 1104, which has an impulse different from that ofpulse 1102.

The pixel is now driven to gray level 6. Since this is a light graylevel, it must, by rule (a) be reached by a black-going pulse.Accordingly, application of rules (a) and (c) requires that this level3/level 6 transition be effected by a two-pulse sequence, namely a firstwhite-going pulse 1106, which drives the pixel white (level 7), followedby a second black-going pulse 1108, which drives the pixel from level 7to the desired level 6.

The pixel is next driven to gray level 4. Since this is a light graylevel, by an argument exactly similar to that employed for the level1/level 3 transition discussed earlier, the level 6/level 4 transitionis effected by a single black-going pulse 1110. The next transition isto level 3. Since this is a dark gray level, by an argument exactlysimilar to that employed for the level 3/level 6 transition discussedearlier, the level 4/level 3 transition is handled by a two-pulsesequence, namely a first black-going pulse 1112, which drives the pixelblack (level 0), followed by a second white-going pulse 1114, whichdrives the pixels from level 0 to the desired level 3.

The final transition shown in FIG. 11A is from level 3 to level 1. Sincelevel 1 is a dark gray level, it must, according to rule (a) beapproached by a white-going pulse. Accordingly, applying rules (a) and(c), the level 3/level 1 transition must be handled by a three-pulsesequence comprising a first white-going pulse 1116, which drives thepixel white (level 7), a second black-going pulse 1118, which drives thepixel black (level 0), and a third white-going pulse 1120, which drivesthe pixel from black to the desired level 1 state.

FIG. 11B shows an odd pixel effecting the same 0-1-3-6-4-3-1 sequence ofgray states as the even pixel in FIG. 11A. It will be seen, however,that the pulses sequences employed are very different. Rule (b) requiresthat level 1, a dark gray level, be approached by a black-going pulse.Hence, the 0-1 transition is effected by a first white-going pulse 1122,which drives the pixel white (level 7), followed by a black-going pulse1124, which drives the pixel from level 7 to the desired level 1. The1-3 transition requires a three-pulse sequence, a first black-goingpulse 1126, which drives the pixel black (level 0), a second white-goingpulse 1128, which drives the pixel white (level 7), and a thirdblack-going pulse 1130, which drives the pixel from level 7 to thedesired level 3. The next transition is to level 6 is a light graylevel, which according to rule (b) is approached by a white-going pulse,the level 3/level 6 transition is effected by a two-pulse sequencecomprising a black-going pulse 1132, which drives the pixel black (level0), and a white-going pulse 1134, which drives the pixel to the desiredlevel 6. The level 6/level 4 transition is effected by a three-pulsesequence, namely a white-going pulse 1136, which drives the pixel white(level 7), a black-going pulse 1138, which drives the pixel black (level0) and a white-going pulse 1140, which drives the pixel to the desiredlevel 4. The level 4/level transition 3 transition is effected by atwo-pulse sequence comprising a white-going pulse 1142, which drives thepixel white (level 7), followed by a black-going pulse 1144, whichdrives the pixel to the desired level 3. Finally, the level 3/level 1transition is effected by a single black-going pulse 1146.

It will be seen from FIGS. 11A and 11B that this drive scheme ensuresthat each pixel follows a “sawtooth” pattern in which the pixel travelsfrom black to white without change of direction (although obviously thepixel may rest at any intermediate gray level for a short or longperiod), and thereafter travels from white to black without change ofdirection. Thus, rules (c) and (d) above may be replaced by a singlerule (e) as follows:

-   -   (e) once a pixel has been driven from one extreme optical state        (i.e., white or black) towards the opposed extreme optical state        by a pulse of one polarity, the pixel may not receive a pulse of        the opposed polarity until it has reached the aforesaid opposed        extreme optical state.

Thus, this drive scheme ensures that a pixel can only undergo, at most,a number of transitions equal to (N−1)/2 transitions, where N is thenumber of gray levels, before being driven to one extreme optical state;this prevents slight errors in individual transitions (caused, forexample, by unavoidable minor fluctuations in voltages applied bydrivers) accumulating indefinitely to the point where serious distortionof a gray scale image is apparent to an observer. Furthermore, thisdrive scheme is designed so that even and odd pixels always approach agiven intermediate gray level from opposed directions, i.e., the finalpulse of the sequence is white-going in one case and black-going in theother. If a substantial area of the display, containing substantiallyequal numbers of even and odd pixels, is being written to a single graylevel, this “opposed directions” feature minimizes flashing of the area.

For reasons similar to those discussed above relating to other driveschemes which divide pixels into two discrete groups, when implementingthe sawtooth drive scheme of FIGS. 11A and 11B, careful attention shouldbe paid to the arrangements of the pixels in the even and odd groups.This arrangement will desirably ensure that any substantially contiguousarea of the display will contain a substantially equal number of odd andeven pixels, and that the maximum size of a contiguous block of pixelsof the same group is sufficiently small not to be readily discernable byan average observer. As already discussed, arranging the two groups ofpixels in a checkerboard pattern meets these requirements. Stochasticscreening techniques may also be employed to arrange the pixels of thetwo groups.

However, in this sawtooth drive scheme, use of a checkerboard patterntends to increase the energy consumption of the display. In any givencolumn of such a pattern, adjacent pixels will belong to oppositegroups, and in a contiguous area of substantial size in which all pixelsare undergoing the same gray level transition (a not uncommonsituation), the adjacent pixels will tend to require impulses ofopposite polarity at any given time. Applying impulses of oppositepolarity to consecutive pixels in any column requires discharging andrecharging the column (source) electrodes of the display as each newline is written. It is well known to those skilled in driving activematrix displays that discharging and recharging column electrodes is amajor factor in the energy consumption of a display. Hence, acheckerboard arrangement tends to increase the energy consumption of thedisplay.

A reasonable compromise between energy consumption and the desire toavoid large contiguous areas of pixels of the same group is to havepixels of each group assigned to rectangles, the pixels of which all liein the same column but extend for several pixels along that column. Withsuch an arrangement, when rewriting areas having the same gray level,discharging and recharging of the column electrodes will only benecessary when shifting from one rectangle to the next. Desirably, therectangles are 1×4 pixels, and are arranged so that rectangles inadjacent columns do not end on the same row, i.e., the rectangles inadjacent columns should have differing “phases”. The assignment ofrectangles in columns to phases may be effected either randomly or in acyclic manner.

One advantage of the sawtooth drive scheme shown in FIGS. 11A and 11B isthat any areas of the image which are monochrome are simply updated witha single pulse, either black to white or white to black, as part of theoverall updating of the display. The maximum time taken for rewritingsuch monochrome areas is only one-half of the maximum time for rewritingareas which require gray to gray transitions, and this feature can beused to advantage for rapid updating of image features such ascharacters input by a user, drop-down menus etc. The controller cancheck whether an image update requires any gray to gray transitions; ifnot, the areas of the image which need rewriting can be rewritten usingthe rapid monochrome update mode. Thus, a user can have fast updating ofinput characters, drop-down menus and other user-interaction features ofthe display seamlessly superimposed upon a slower updating of a generalgrayscale image.

As discussed in the aforementioned copending application Ser. Nos.09/561,424 and 09/520,743, in many electro-optic media, especiallyparticle-based electrophoretic media, it is desirable that the drivescheme used to drive such media be direct current (DC) balanced, in thesense that, over an extended period, the algebraic sum of the currentspassed through a specific pixel should be zero or as close to zero aspossible, and the drive schemes of the present invention should bedesigned with this criterion in mind. More specifically, look-up tablesused in the present invention should be designed so that any sequence oftransitions beginning and ending in one extreme optical state (black orwhite) of a pixel should be DC balanced. From what has been said above,it might at first appear that such DC balancing may not be achievable,since the impulse, and thus the current through the pixel, required forany particular gray to gray transition is substantially constant.However, this is only true to a first approximation, and it has beenfound empirically that, at least in the case of particle-basedelectrophoretic media (and the same appears to be true of otherelectro-optic media), the effect of (say) applying five spaced 50 msecpulses to a pixel is not the same as applying one 250 msec pulse of thesame voltage. Accordingly, there is some flexibility in the currentwhich is passed through a pixel to achieve a given transition, and thisflexibility can be used to assist in achieving DC balance. For example,the look-up table used in the present invention can store multipleimpulses for a given transition, together with a value for the totalcurrent provided by each of these impulses, and the controller canmaintain, for each pixel, a register arranged to store the algebraic sumof the impulses applied to the pixel since some prior time (for example,since the pixel was last in a black state). When a specific pixel is tobe driven from a white or gray state to a black state, the controllercan examine the register associated with that pixel, determine thecurrent required to DC balance the overall sequence of transitions fromthe previous black state to the forthcoming black state, and choose theone of the multiple stored impulses for the white/gray to blacktransition needed which will either accurately reduce the associatedregister to zero, or at least to as small a remainder as possible (inwhich case the associated register will retain the value of thisremainder and add it to the currents applied during later transitions).It will be apparent that repeated applications of this process canachieve accurate long term DC balancing of each pixel.

It should be noted that the sawtooth drive scheme shown in FIGS. 11A and11B is well adapted for use of such a DC balancing technique, in thatthis drive scheme ensures that only a limited number of transitions canelapse between successive passes of any given pixel through the blackstate, and indeed that on average a pixel will pass through the blackstate on one-half of its transitions.

The objectionable effects of reset steps may be further reduced by usinglocal rather than global updating, i.e., by rewriting only thoseportions of the display which change between successive images, theportions to be rewritten being chosen on either a “local area” or apixel-by-pixel basis. For example, it is not uncommon to find a seriesof images in which relatively small objects move against a larger staticbackground, as for example in diagrams illustrating movement of parts inmechanical devices or diagrams used in accident reconstruction. To uselocal updating, the controller needs to compare the final image with theinitial image and determine which area(s) differ between the two imagesand thus need to be rewritten. The controller may identify one or morelocal areas, typically rectangular areas having sides aligned with thepixel grid, which contain pixels which need to be updated, or may simplyidentify individual pixels which need to be updated. Any of the drivescheme already described may then be applied to update only the localareas or individual pixels thus identified as needing rewriting. Such alocal updating scheme can substantially reduce the energy consumption ofa display.

The aforementioned drive schemes may be varied in numerous waysdepending upon the characteristics of the specific electro-optic displayused. For example, in some cases it may be possible to eliminate many ofthe reset steps in the drives schemes described above. For example, ifthe electro-optic medium used is bistable for long periods (i.e., thegray levels of written pixels change only very slowly with time) and theimpulse needed for a specific transition does not vary greatly with theperiod for which the pixel has been in its initial gray state, thelook-up table may be arranged to effect gray state to gray statetransitions directly without any intervening return to a black or whitestate, resetting of the display being effected only when, after asubstantial period has elapsed, the gradual “drift” to pixels from theirnominal gray levels could have caused noticeable errors in the imagepresented. Thus, for example, if a user was using a display of thepresent invention as an electronic book reader, it might be possible todisplay numerous screens of information before resetting of the displaywere necessary; empirically, it has been found that with appropriatewaveforms and drivers, as many as 1000 screens of information can bedisplayed before resetting is necessary, so that in practice resettingwould not be necessary during a typical reading session of an electronicbook reader.

It will readily be apparent to those skilled in display technology thata single apparatus of the present invention could usefully be providedwith a plurality of different drive schemes for use under differingconditions. For example, since in the drive schemes shown in FIGS. 9 and10, the reset pulses consume a substantial fraction of the total energyconsumption of the display, a controller might be provided with a firstdrive scheme which resets the display at frequent intervals, thusminimizing gray scale errors, and a second scheme which resets thedisplay only at longer intervals, thus tolerating greater gray scaleerrors but reduce energy consumption. Switching between the two schemescan be effected either manually or dependent upon external parameters;for example, if the display were being used in a laptop computer, thefirst drive scheme could be used when the computer is running on mainselectricity, while the second could be used while the computer wasrunning on internal battery power.

From the foregoing description, it will be seen that the presentinvention provides a driver for controlling the operation ofelectro-optic displays, which are well adapted to the characteristics ofbistable particle-based electrophoretic displays and similar displays.

From the foregoing description, it will also be seen that the presentinvention provides a method and controller for controlling the operationof electro-optic displays which allow accurate control of gray scalewithout requiring inconvenient flashing of the whole display to one ofits extreme states at frequent intervals. The present invention alsoallows for accurate control of the display despite changes in thetemperature and operating time thereof, while lowering the powerconsumption of the display. These advantages can be achievedinexpensively, since the controller can be constructed from commerciallyavailable components.

In the remnant voltage method of the present invention, measurement ofthe remnant voltage is desirably effected by a high impedance voltagemeasurement device, for example a metal oxide semiconductor (MOS)comparator. When the display is one having small pixels, for example a100 dots per inch (DPI) matrix display, in which each pixel has an areaof 10′ square inch or about 6×10′ mm², the comparator needs to have anultralow input current, as the resistance of such a single pixel is ofthe order of 10¹² ohm. However, suitable comparators are readilyavailable commercially; for example, the Texas Instruments INA111 chipis suitable, as it has an input current on only about 20 pA.(Technically, this integrated circuit is an instrumentation amplifier,but if its output is routed into a Schmitt trigger, it acts as acomparator.) For displays having large single pixels, such as largedirect-drive displays (defined below) used in signage, where theindividual pixels may have areas of several square centimeters, therequirements for the comparator are much less stringent, and almost anycommercial FET input comparator may be used, for example the LF311comparator from National Semiconductor Corporation.

It will readily be apparent to those skilled in the art of electronicdisplays that, for cost and other reasons, mass-produced electronicdisplays will normally have drivers in the form of application specificintegrated circuits (ASIC's), and in this type of display the comparatorwould typically be provided as part of the ASIC. Although this approachwould require provision of feedback circuitry within the ASIC, it wouldhave the advantage of making the power supply and oscillator sections ofthe ASIC simpler and smaller in area. If tri-level general image flowdrive is required, this approach would also make the driver section ofthe ASIC simpler and smaller in area. Thus, this approach wouldtypically reduce the cost of the ASIC.

Conveniently, a driver which can apply a driving voltage, electronicallyshort or float the pixel, is used to apply the driving pulses. Whenusing such a driver, on each addressing cycle where DC balancecorrection is to be effected, the pixel is addressed, electronicallyshorted, then floated. (The term “addressing cycle” is used herein inits conventional meaning in the art of electro-optic displays to referto the total cycle needed to change from a first to a second image onthe display. As noted above, because of the relatively low switchingspeeds of electrophoretic displays, which are typically of the order oftens to hundreds of milliseconds, a single addressing cycle may comprisea plurality of scans of the entire display.) After a short delay time,the comparator is used to measure the remnant voltage across the pixel,and to determine whether it is positive or negative in sign. If theremnant voltage is positive, the controller may slightly extend theduration of (or slightly increase the voltage of) negative-goingaddressing pulses on the next addressing cycle. If, however, the remnantvoltage is negative, the controller may slightly extend the duration of(or slightly increase the voltage of) positive-going voltage pulses onthe next addressing cycle.

Thus, the remnant voltage method of the invention places theelectro-optic medium into a bang-bang feedback loop, adjusting thelength of the addressing pulses to drive the remnant voltage towardzero. When the remnant voltage is near zero, the medium exhibits idealperformance and improved lifetime. In particular, use of the presentinvention may allow improved control of gray scale. As noted earlier, ithas been observed that the gray scale level obtained in electro-opticdisplays is a function not only of the starting gray scale level and theimpulse applied, but also of the previous states of the display. It isbelieved (although this invention is in no way limited by this belief)that one of the reasons for this “history” effect on gray scale level isthat the remnant voltage affects the electric field experienced by theelectro-optic medium; the actual electric field influencing the behaviorof the medium is the sum of the voltage actually applied via theelectrodes and the remnant voltage. Thus, controlling the remnantvoltage in accordance with the present invention ensures that theelectric field experienced by the electro-optic medium accuratelycorresponds to that applied via the electrodes, thus permitting improvedcontrol of gray scale.

The remnant voltage method of the present invention is especially usefulin displays of the so-called “direct drive” type, which are divided intoa series of pixels each of which is provided with a separate electrode,the display further comprising switching means arranged to controlindependently the voltage applied to each separate electrode. Suchdirect drive displays are useful for the display of text or otherlimited character sets, for example numerical digits, and are describedin, inter alia, the aforementioned International application PublicationNo. 00/05704. However, the remnant voltage method of the presentinvention can also be used with other types of displays, for exampleactive matrix displays which use an array of transistors, at least oneof which is associated with each pixel of the display. Activating thegate line of a thin film transistor (TFT) used in such an active matrixdisplay connects the pixel electrode to the source electrode. Theremnant voltage is small compared to the gate voltage (the absolutevalue of the remnant voltage typically does not exceed about 0.5 V), sothe gate drive voltage will still turn the TFT on. The source line canthen be electronically floated and connected to a MOS comparator, thusallowing reading the remnant voltage of each individual pixel of theactive matrix display.

It should be noted that, although the remnant voltage on a pixel of anelectrophoretic display does closely correlate with the extent to whichthe current flow through that pixel has been DC-balanced, zero remnantvoltage does not necessarily imply perfect DC-balance. However, from thepractical point of view, this makes little difference, since it appearsto be the remnant voltage itself rather than the DC-balance historywhich is responsible for the adverse effects noted herein.

It will readily be apparent to those skilled in the display art that,since the purpose of the remnant voltage method of the present inventionis to reduce remnant voltage and DC imbalance, this method need not beapplied on every addressing cycle of a display, provided it is appliedwith sufficient frequency to prevent a long-term build-up of DCimbalance at a particular pixel. For example, if the display is onewhich requires use of a “refresh” or “blanking” pulse at intervals, suchthat during the refresh or blanking pulse all of the pixels are drivento the same display state, normally one of the extreme display states(or, more commonly, all of the pixels are first driven to one extremedisplay state, and then to the other extreme display state), the methodof the present invention might be practiced only during the refresh orblanking pulses.

Although the remnant voltage method of the invention has primarily beendescribed in its application to encapsulated electrophoretic displays,this method may be also be used with unencapsulated electrophoreticdisplays, and with other types of display, for example electrochromicdisplays, which display a remnant voltage.

From the foregoing description, it will be seen that the remnant voltagemethod of the present invention provides a method for drivingelectrophoretic and other electro-optic displays which reduces the costof the equipment needed to ensure DC balancing of the pixels of thedisplay, while providing increasing display lifetime, operating windowand long-term display optical performance.

Numerous changes and modifications can be made in the preferredembodiments of the present invention already described without departingfrom the spirit and skill of the invention. Accordingly, the foregoingdescription is to be construed in an illustrative and not in alimitative sense.

1. A method of driving a bistable electrophoretic display having atleast one pixel with two extreme optical states, the electrophoreticdisplay containing electrophoretically-mobile particles suspended in aliquid suspension medium, the method comprising: (a) driving the pixelfrom an initial gray level to one extreme optical state different fromthe initial gray level; and (b) immediately driving the pixel from theone extreme optical state to the opposed extreme optical state andimmediately thereafter driving the pixel to a final gray level differentfrom the one extreme optical state.
 2. A method according to claim 1wherein the display is a microcell display with theelectrophoretically-mobile particles and the suspension medium retainedwithin a plurality of cavities formed in a carrier medium.
 3. A methodaccording to claim 18 wherein the display is an encapsulatedelectrophoretic display comprising a plurality of capsules, each ofwhich itself comprises an internal phase containing theelectrophoretically-mobile particles and the suspension medium, and acapsule wall surrounding the internal phase.
 4. A bistableelectrophoretic display having at least one pixel with two extremeoptical states, the electrophoretic display containingelectrophoretically-mobile particles suspended in a liquid suspensionmedium, and a display controller for applying electric field to thepixel and thereby changing the optical state thereof from an initialgray level to a final gray level, the display controller being arrangedto: (a) drive the pixel from the initial gray level to one extremeoptical state different from the initial gray level; and (b) immediatelydrive the pixel from the one extreme optical state to the opposedextreme optical state and immediately thereafter drive the pixel to afinal gray level different from the one extreme optical state.
 5. Adisplay according to claim 4 wherein the display is a microcell displaywith the electrophoretically-mobile particles and the suspension mediumretained within a plurality of cavities formed in a carrier medium.
 6. Adisplay according to claim 4 wherein the display is an encapsulatedelectrophoretic display comprising a plurality of capsules, each ofwhich itself comprises an internal phase containing theelectrophoretically-mobile particles and the suspension medium, and acapsule wall surrounding the internal phase.
 7. A method of driving abistable electrophoretic display having at least one pixel with twoextreme optical states, the electrophoretic display containingelectrophoretically-mobile particles suspended in a liquid suspensionmedium, the method comprising: (a) driving the pixel from an initialgray level to one extreme optical state different from the initial graylevel; and (b) driving the pixel from the one extreme optical state tothe opposed extreme optical state and thereafter driving the pixel to afinal gray level different from the one extreme optical state, whereinstep (b) is effected by driving the pixel immediately the one extremeoptical state to the opposed extreme optical state, immediatelyhereafter driving the pixel back to the one extreme optical state andimmediately thereafter driving the pixel to the final gray level.
 8. Abistable electrophoretic display having at least one pixel with twoextreme optical states, the electrophoretic display containingelectrophoretically-mobile particles suspended in a liquid suspensionmedium, and a display controller for applying electric field to thepixel and thereby changing the optical state thereof from an initialgray level to a final gray level, the display controller being arrangedto: (a) drive the pixel from the initial gray level to one extremeoptical state different from the initial gray level; and (b) drive thepixel from the one extreme optical state to the opposed extreme opticalstate and thereafter drive the pixel to a final gray level differentfrom the one extreme optical state, wherein the display controller isarranged to effect (b) by driving the pixel immediately from the oneextreme optical state to the opposed extreme optical state, immediatelythereafter driving the pixel back to the one extreme optical state andimmediately thereafter driving the pixel to the final gray level.